(Note: If anyone wants to add specifics on crunch to my answer, or copy-paste my answer to use as the basis for a better answer, please do so. I don't know enough to be helpful!)
I presume your approach is this: You want to try to combine the 3.0 core rulebooks with the 3.5 SRD, presumably because you want to use the 3.0 books for source material and advice but the SRD for stat blocks and an authoritative system of rules. You want to know what will you miss, meaning what could trip you up because you missed the difference between 3.0 and 3.5. I'm going to explain from a GM's perspective, that is, that you'll be planning a campaign, and that you've familiarized yourself with 3.5 already.
In short, as long as you remember to always treat the SRD as the only source for crunch, you won't miss much. A very small amount of mechanical data is missing from the 3.5 SRD (such as the stat blocks for the few monsters that Wizards kept out of the SRD, and the PHB's XP table). You can use the old versions safely, when necessary, provided you're mindful of systemic differences (such as the change to Damage Reduction). It'll be easy to know when to be careful, though, because you'll be looking at a book instead of a website.
Let me dig in to each core rulebook for specifics.
Player's Handbook. Honestly, just tell the players not to trust the 3.0 PHB and you'll be fine. Everything that isn't crunch is common cultural knowledge at this point, such as knowing that fantasy dwarves are good miners and like to use axes. The XP table is all that's missing, I believe, and it didn't change.
Dungeon Master's Guide. I actually own the 3.0 DMG but not the 3.5 DMG, even though I haven't GMed 3.0 in years. In fact, I reference it all the time for my Pathfinder game, for its general tools and advice on how to GM. So I think you'll do very well without the 3.5 DMG and your players will never notice a difference, provided you always use the SRD for actual game mechanics similar material such as the magic item chapter. But, say, the table of 100 plot ideas will never be out of date, you know?
Monster Manual. Effectively you'll be ignoring it wholesale except for, e.g., reading the colorful monster descriptions aloud to the players, and that's safe since they didn't redesign any monsters in terms of concept (that I know of). There is the matter of the non-SRD monsters, but again, you'll know to be careful when using them, so you'll be fine.
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.
Best Answer
No, going by the first amazon review the board game is a 4.0 product, and therefore it cannot be the same as the 3.5 Castle Ravenloft book.