The relationship between the two adventures is fascinating and confusing. The short answer is: No, they are not very similar, at least not in a way that ruins or spoils one adventure for someone who's played the other.
On Castle Ravenloft itself:
Expedition and Curse of Strahd both reuse the original Ravenloft map. Although each version has its own tricks and monsters and treasure, they really are the same dungeon in most ways. Except one big one!
Big Expedition to Castle Ravenloft Spoiler:
Location K89, the subcatacomb with the prehistoric altar, does not appear in Curse of Strahd (or, I think, any other version of the castle). Having studied Curse of Strahd very closely before reading through Expedition, I thought this "new" location was really cool, and I think you could lean on it as a source of real surprise for your players who've already seen the castle.
On the structure of the two adventures:
The zombie outbreak and shrines of Expedition do not feature in Curse of Strahd. Curse of Strahd is meant to be fairly open-ended (as 5E D&D adventures go) but the generally assumed "critical path" is limited to searching Barovia for the Tome of Strahd, Sunsword, and Holy Symbol of Ravenkind before facing Strahd himself. Since the artifacts are placed randomly in the Tarokka reading, it's possible for any given group to "skip" a lot of the locations and content.
So your players who've played Curse of Strahd are sure to have a new adventure in Expedition to Castle Ravenloft. That being said, there are some inconsistencies and near-consistencies that you may want to watch out for.
Curse of Strahd contains a lot of Easter egg-like references to previously published Ravenloft adventures, and one could make a case that Curse "takes place after" I6, Expedition, and 4E's Fair Barovia. But it's not that simple. The name "Kavan" makes for a good example:
In Expedition to Castle Ravenloft, Kavan is a vampire barbarian who serves Strahd. In Curse of Strahd, players can find the "Blood Spear of Kavan," a magic item containing the spirit of an eponymous blood-drinking barbarian—but the book explains that this Kavan was buried in the valley before Strahd ever arrived. So veterans of Expedition are supposed to recognize this spear (and veterans of Curse might recognize Kavan himself, if they happened to find that spear) but they're not exactly the same character.
This kinda-sorta-continuity is characteristic of many people and places throughout Curse of Strahd's Barovia.
If your players are Legend of Zelda fans, this kind of thing is nothing new to them. If you're worried that they'll be confused, you can warn them that Expedition and Curse act like alternate universes to each other. In either case, I think that having already visited a version of Barovia won't diminish (and will tend to enhance) their experience of Expedition.