Personally, I think spells are going to be where a lot of the pain points might arise when you start to run pre and post revision material together at a table. Making an early judgment from feats, they're going to be quite willing to have the same named object (whether that be a feat name or a spell name) use very different rules. And what makes spells distinct is that they aren't personal to the character, they have an in-fiction existence that's shared among all the characters, and the NPCs as well.
So DMs are going to have to say "Ok, we're only going to use the revised version of the spells at this table, but we'll still use the old versions of spells X, Y, and Z because I like those versions better." It's not a case where the guy happily playing his half-elf wizard from 2014 can just use the spells from his 2014 book without causing some ripples at the table.