Up until now, I only really payed attention to quality enchantement and was thus under the Impression that it was possible to simply override an existing enchantment with a new one. Now, wanting to give my unique pike the elemental enchantment that I wanted for it (corrosive, since the spear is green too) I had to discover that it was in fact not possible to change it and I either have to play with a silly looking weapon for the rest of the playthrough, replay about 5-7 hours or enable cheats in my first potd game.
If it is really seen as necessary to put such a massively inconsistent system into the game, one would imagine that it would at least be a good idea to, you know, tell anybody about it, instead of teaching them in the school of "ha, gotcha!".