It could be a stand alone adventure for a higher level party. The original Ravenloft was made as a one session adventure played on Halloween. I was going to run the original Ravenloft for my home game before I found out about CoS.
I would think the players should be level 8 or so before entering Barovia if they are planning to go right to the castle. If they can still get the sword and amulet before fighting Strahd, they should be fine if they level up once or twice while exploring the castle.
For a one day (a very long day) campaign you can go with:
Enter Barovia
Find the village of Barovia
Hear Mad Mary
Blood of the Vine Tavern and Bildrath's Mercantile can stay the same (except Ismark is not trying to get his sister to Vallaki).
Burgomaster's House (meet Ireena)
Bury Burgomaster (don't have the vampire spawn in basement with the players being higher level)
Visit Madam Eva (remove the ally card and any non-castle locations for tome/sword/amulet)
Have the carriage be available at the crossroads at area I on the Barovian Map. Have the gate to the west stay closed, or just have nothing interesting beyond it.
Changes in the castle:
Room K78 (Brazier Room): Remove the gems that send the players outside the castle. That only leaves 3 gems.
Ignore references to things outside the castle (dragon skull, banshee, werewolf, crypt 11 holy symbol, and others I am forgetting)