** SPOILERS **
A large portion of CoS is a re-do of Castle Ravenloft (i.e. the castle itself). Let's call if 30%. If they're intimately familiar with the castle, the location of its secret doors, how the crypts and teleporters work, and the general location of some of the chief antagonists... then, yeah, that bit will seem very familiar to them. Strahd himself is much the same as well, although (as with the original) he has a variety of different personal goals he could be pursuing.
The majority of the module, however, is other locations in (the 5e version of) Barovia. There are a bunch of these, and each is good for about 1 session. Think of them as mini-dungeons with (average) 3-5 encounters each. Some are significantly bigger. Examples include: werewolf caves, a monastery, a ruined fortress, druid grove, hag den, etc.
There's a particular large and dangerous dungeon which seems quite out of place with the rest of the module, along with Strahd's dangerous new butler (an elf called Rahadin). This appears to be a nod to the classic module B7 - Rahasia, which was penned by Ravenloft's original author Tracy Hickman. Rahasia revolved around elves corrupted by a triad of witches trapped in crystal prisons. There was a villain called the Rahab. It had a multi-level dungeon, with frequent statues of robed monk-like figures, and a maze-like design with many alcoves and side-rooms. You'll see all of those elements repeated in the new dungeon in CoS, and it clearly influences the design of Rahadin as well. Some reviewers have felt the new dungeon is inconsistent with previous treatments of Ravenloft, especially given it's implied treatment of the Dark Powers. However, if you don't want to use it, it's only 10% of the book content and has no bearing on the core story.
Overall, I think CoS is an excellent resource. It's very sand-boxy, more so than most WotC modules. You can use all the various elements (random goals, vivid NPCs, many mini-dungeons) to craft the sort of story you want to tell. The Castle itself will probably still be a major element, and will seem very familiar to veterans, but there's many sessions of adventure outside the castle gates. In my opinion and for the preferences of my table, it's probably one of the best of the 5e WotC modules. However, it's worth noting that I don't particularly care for continuity with older versions of Ravenloft. If you truly love the 2e vision, and would be offended by a different approach to the material, be cautious. There are elements here (Rahadin, an interpretation of the Dark Powers, shadow elves, extra-planar creatures) that never existed in older versions.