Culture conversion has always been a sort of silly feature you do for funsies, so it doesn't seem like a very big deal. I've seen the AI commit some fairly revolting cultural atrocities (Castilian Italy is probably the most common), and being able to reverse that at a heavy discount sounds like a fine feature. Converting cultures to ones other than your own does seem very unrealistic, but I can imagine having some fun with it.
It really isn't the change I'd be looking for, though. Something I'd like to see is a system where you can select an unaccepted culture to "represent" its group, simulating the way empires often favored a minority in local leadership positions as a way of creating a loyal and ruthless administration (allowing the player to set up the same sort of messed up colonial structures that caused so many modern problems). Or really, just any way of getting reduced maluses when you have, for instance, the entirety of China but accept none of the cultures because the sum of each individual culture prevents any other one from being accepted. Being able to get an entire culture group accepted to the point where it gives the same level of maluses you get from unaccepted cultures in your own group would be a great improvement.