I still believe they actually were supposed to be the "Unbirth" for a few reasons, the two big ones being:
1. It's Birth by Sleep. I don't think that's a coincidence. By the time we got the finished product, really, the game has hardly anything to do with that term at all. It's only mentioned in the secret ending, and only there as a vague concept. You would figure Nomura would have a better reason for that choice.
2. Nomura has already admitted that he essentially dumbed the game down because he thought some of the more Eastern concepts would not be understood by Western fans. For instance, Sora was supposed to have connected with Ven's heart, in Nomura's original conception, before he was born. You know. Pre-birth. Unborn so to speak. Instead, he altered this so that he connected to Ven when he was born.
So my thinking is that if Nomura really thought we were too stupid to understand that, then if the enemies themselves were based around a similar concept, he may have opted to change their entire nature. And given that "Unversed" and "Unbirth" are homonyms in the Japanese language, he made the swap. I mean, really, the reason they gave for their names is probably one of the dumbest things to come out of this series. WE DON'T KNOW MUCH ABOUT THEM SO WE WILL CALL THEM UNVERSED. Which is a damn shame because they ended up being nothing more than pointless filler enemies.
Also, on topic:
Dis-soul