I purposely did a clean install of D4 (nuking the /Data folder and clicking Update in the Battle.net app) so my install could be contiguous. VoH’s patch would have overwritten so much of the previous data that enumeration would be an issue with CASC so I decided to avoid that by just downloading the entire game, including the high resolution assets.
Just remember that when you log in you may want to sit in the character select screen for a few minutes while shaders are downloaded and compiled, otherwise you’ll get major hitching in the game world for a while.