Before there was Curse of Strahd (CoS), possibly the most famous 5e adventure, there was the 3rd edition adventure Expedition to Castle Ravenloft. Instead of running CoS, I sometimes run Expedition. It's fun for players who have never experienced Ravenloft before, and it's got a few surprises for players already familiar with Curse of Strahd. I'm reviewing the 3rd edition book, chapter by chapter. There will be minor spoilers in these posts, so this is primarily intended for Dungeon Masters.
Chapter 3 is all about the lands of Barovia.
The introduction to the chapter provides an overview of the land surrounding the central village, and most importantly introduces the three hags. Players of CoS will think these three hags are Morgantha, Bella Sunbane, and Offalia Wormwiggle, but they're not. They don't live in a windmill, they're not night hags. They're entirely different people, and I won't betray their identities or exact roles on the story here. It's fun, though, to see players adjust expectations as it becomes clear that the hags they're hearing about aren't the ones they've been imagining.
A word of warning: there is no windmill in this adventure unless you add one in. I mention this because if you run Death House as an introductory adventure, as I have in the past, then you need to either change the deed to Old Bonegrinder to something else or be sure to put Old Bonegrinder in Barovia somewhere. Six hags in one adventure seems a bit much, so I wouldn't inhabit the windmill with hags, but there are plenty of other threats you could add in, if you wanted to make it an encounter, or you could make in a benign headquarters for the PCs, or whatever.
The crossroads location is a literal crossroads that characters encounter because it inevitably lies between Barovia, Castle Ravenloft, and Tser Pool. At the crossroads, players meet Sir Urik, a Knight of the Raven. In the lore of Ravenloft, the Knights of the Raven are an ancient order of noble fighters sworn to defeat evil wherever it may appear. Unfortunately for the order, Strahd appeared in Ravenloft and dominated the land and its people, and so the order's numbers have dwindled to one or two members. Mechanically, a player character meeting specific requirements can join the order, taking on a prestige class and gaining new abilities, including the very cool ability to smite undead (a turn undead ability that smites instead of turns) and a raven companion.
As with the Lightbringers, I want to adapt this for 5e but I have nothing organized enough to post as of this writing. You could either adapt the features yourself, or you can just use the Knights of the Raven as flavour, maybe with a token benefit here or there (maybe a slight boost to a cleric's turn undead ability to make it more lethal, and a raven sidekick according to the rules described in Tasha's Cauldron).
Sir Urik doesn't play much of a role in the story aside from inducting interested PCs into the order. Compared to Ashlyn, Sir Urik is a little undeveloped, and I'm not entirely sure where he's meant to fit in. In fact, the book even suggests using Sir Urik as an easy replacement for a dead character.
Unlike Ashlyn, Sir Urik isn't given a goal, so it's hard to get a feel for what his threshold is when he decides to help the PCs. Ashlyn's immediate goal is to find her missing soldiers, so she helps PCs as long as they're going her way, and once she finds them she stops helping. That's pretty clear. Sir Urik has no goal. He's the last of the Knights of the Raven, so he fights evil wherever it appears, and obviously evil is all over Ravenloft. To me, it seems like Sir Urik should jump at the chance to join a party of PCs, but there's no way I'm going to play an NPC and his raven companion for the rest of the adventure.
To deal with the vagueness of Sir Urik's purpose, I say that he's looking for Lady Vay Rallen, an eminent Knight of the Raven who is said to still walk in the lands of Ravenloft. As the Dungeon Master, you'll know that Lady Vay Rallen is long dead, held captive within the walls of Castle Ravenloft, and the players are likely to find that out later. The effect is that Sir Urik has an excuse not to roam the lands with the players (because whatever way they're heading is not the way he's heading), and it gives the players a little mini-quest that they'll end up solving just by exploring the castle later in the adventure.
There are some monsters that dwell around the crossroads called murks, which aren't in the 5e Monster Manual. Their key feature is Wisdom drain, with the penalty of 1 negative level should a PC's Wisdom reach 0. You could try to emulate that in 5e, but negative levels can lead to a lot of bookkeeping, which then gets reversed once greater restoration is cast anyway. I find it easier to just use a wight (if you don't mind a corporeal creature) or a ghost or a few shadows, or a scarecrow (how often do you get to use those?) or a combination thereof. (There's a wraith encounter elsewhere, so those are already spoken for.)
Elsewhere, there's a possible encounter with a CR 9 creature called the Caller in Darkness. It's incorporeal and psionics-based magical cloud with spells that mess with the player character's mind (ego whip, for instance.) As a replacement, the Gallows Speaker from Van Richten's Guide to Ravenloft is a relatively close match. It's incorporeal, and has an ability to throw off your next attack, or to cause psychic damage. Flavourfully, it's a definite match. Its CR is lower than the Caller, so you might need to add something else spooky to the encounter, but in Ravenloft I think it's never not appropriate to throw in an extra death's head, ghost, jiangshi, shadow, vampiric mist, zombie (or practically anything from Van Richten's Guide) as needed.
There are likely a few other monsters requiring swaps that I'm not remembering now, but there are far more that don't require a swap at all. Just look up the name of the monster in your current edition's bestiary and use updated stats. It won't slow you down a bit.
The most famous mechanic of Ravenloft is that of the Tarokka deck, and how it determines the specifics of the story. It's important to get the player characters to Madam Eva pretty early on, and that's why most of the townsfolk in Barovia keep telling them to go talk to her. Chapter 3 has all the information you need on Madam Eva and, most importantly, how to do a Tarokka reading.
For Tarokka, you can use a tarot deck, a Three Dragon Ante deck (I don't think they're made any more), or a standard set of playing cards. I have the Fae Deck by Gent Supply Co. It's not an exact match for the theme of Ravenloft, but the deck is fantasy themed with black cards and silver and gold ink, so it's a pretty good match. An actual Tarokka deck didn't exist in 3rd edition, so you would need to adapt the schema for use with a 5e Tarokka deck, but it wouldn't be too difficult. What seems complex and mysterious to players is actually a pretty simple trick for the Dungeon Master. I won't reveal how the fortune telling works here, of course, but it's a lot easier to run than you might imagine. Just remember to keep notes so you remember the results!
To determine the key components of the story, you need only 5 face cards, so you can pick the cards that are the most evocative to you and let them serve as the fonundation for the fortune telling. They set the tone, and they're the cards the players see first. Using a unique deck of cards is nice, because many players will have never seen novelty playing cards before, but you can use anything. You draw two more cards randomly, and these determine important elements of the adventure, many of them depending upon Strahd's secondary goal (established in chapter 1 of the book with a d6 roll, although I use cards for that too.)
Madam Eva is a hugely important character in this book, for many of the same reasons, and for several different ones besides. The Vistani aren't terribly well developed, and it's here I'd probably go so far as to incorporate The Carnival from Van Richten's Guide. In this book, it's said only that this group of Vistani are smugglers and con artists, and you get the sense that the camp is mostly a thieves camp. While that's accurate (they are indeed active smugglers), it limits how much the player characters can do in the camp, which seems like a waste to me. There's a way for the players to befriend Madam Eva, in which case she's actually very helpful toward them, and I think allowing the players to carouse and socialize within the camp can be a useful diversion. Of course, there's also a world where Madam Eva is a dangerous enemy, depending on your players and, as always, the will of the fates (or Tarokka deck, as the case may be.)
There are lots of other locations and subplots, and there's plenty of room for you to make up your own side quests. The book is focused on the main goal, of course, but the setting comes through loud and clear, and there's no shortage of threats or tropes for you to exploit. Chapter 3 is, in a way, surprisingly short, considering the inrush of ideas you'll get from reading through it. Chapter 4, the next and final chapter, is all about Castle Ravenloft itself.