I don't own Curse of Strahd, arguably one of the most famous D&D 5e modules. I love that module, partly because I'm a sucker for horror and also because it's a really good module, and I do own Van Richten's Guide to Ravenloft. However, I'm the happy owner of Expedition to Castle Ravenloft, the 3rd Edition module upon which Curse of Strahd is based.
This wasn't a philosophical or even artistic choice I made, it's just that the 3e version already existed by the time the 5e version came out, and I just didn't see the point in doubling up on essentially the same adventure. (It's not really the same, so read on.)
I'm comfortable converting 3.5 adventures to 5e, and 5e adventures to 3.5. I can usually do the conversion as the game is played, with little to no special preparation aside from reading the module in advance, as usual.
There's a little bonus provided by the old version of the Strahd story that I hadn't anticipated when making the decision not to buy the new book. The setting is the same, of course. And the stories are very similar. But Curse of Strahd gets a lot of attention, from actual play podcasts and streams to one-shot adaptations. It's probably the best known module of modern D&D. I've met people who've never played D&D but are interested in trying it because they want to play Curse of Strahd on Halloween.
So it's pretty nice to have an adventure that mirrors the Curse of Strahd in practically every way, but deviates from it just enough to throw players off guard. Many of the plot points appear to be the same, and yet in the end the goal or the reason behind the quest is different. The Sunsword is still of vital importance, as are some other key artifacts that I won't spoil here, but if a player dives into the 3rd Edition module and immediately goes to seek out the three items from Curse of Strahd, they'll miss out on some of the most important ways of further crippling the famous villain.
Many of the NPCs are also the same-but-different. Instead of Ezmerelda d'Avenir, for instance, there's Ashlyn of the Lightbringers. There are hags in both modules, but their motives and identities differ. Madame Eva and Ireena and Van Richten are constants, but maybe not exactly the same in every way.
It's all familiar enough to lull you into a sense of secturity, and yet it also feels like it exists in a parallel universe. And I can't think of a more fitting, and possibly unsettling, tone for a game in Ravenloft.
Expedition to Castle Ravenloft and Curse of Strahd both feature suggested story hooks for why the player characters end up in Ravenloft. Maybe Strahd summons them himself, or maybe an NPC within Barovia needs their assistance, or maybe they just get lost in the mists one night. Regardless of which hook you go with, I'm a fan of the optional Death House introduction from the 5e version. I'm a fan of haunted house adventures, so Death House for me is a fun and focused way to get players into the spirit of Ravenloft. It works just as well in the 3.5 version as it does in the 5e version.
Alternately, there's the House of Lament adventure, on page 202 of Van Richten's Guide to Ravenloft, which with minor (or no) adjustment can be used as a great intro to Ravenloft.
The 5e version has the Death House, but there's bonus content for the 3e version too (well, sort of). Way back when magazines existed, Dragon issue 351 featured a section on all of the major campaign settings of D&D. Included in this was Sithicus, a domain of dread that formed around a fallen knight from Krynn (yes, of the Dragonlance setting). Like other domains of Dread, Sithicus features a darklord: Inza Magdova Kulchevich, a Vistani rogue. Sithicus is southwest of Barovia, so depending on how you manage the domains of dread in your campaign it could be accessible by player characters. If you isolate each domain with impenetrable mists, then at the very least some of the Vistani could have some tales of Sithicus.
It's not quite a natural fit the way Death House or the House of Lament are, but it's still an interesting addition to the domains of dread, and it's one that's oddly missing from Van Richten's Guide.
People whose first and only exposure to Ravenloft is Curse of Strahd may not be aware that Ravenloft is a whole D&D setting, not just the one module. While Van Richten's Guide to Raveloft does much to bring the breadth of Ravenloft to 5e, it's always fun, and empowering from a story perspective, to realise that previous editions have been developing Ravenloft for decades before Curse of Strahd was released. These can be great resources for your next foray into Ravenloft, whether it's an expedition or a curse.