Spoiler ShowMy guess is the change has to do with how there was a huge dry spot of information and then suddenly the Unversed name spelling was changed. If we go by what we know I imagine it has to do with how originally the plot was going to tackle theme of "the heart exists before a person's birth". Given Ventus can't make Unversed but Vanitas can and Vanitas was clearly affected by Sora in a lot of ways. Vanitas would have literally taken the form of a person who wasn't born yet...My guess is that the intent was "what does an emotion created and felt by someone that has yet to exist look like" and so we have these emotions that don't know how to BE emotions. Represented thusly as a weird sort of energy that either takes a physical form as these unholy monsters, infects objects turning them into emotional monsters, or infects people mingling with their emotions and giving their emotions the same ability to infect objects (like the mirror) or take physical form as monsters (like the pumpkin carriage unversed).
I would agree very much that the Unversed were sanded down to nothing by editing, the Unversed are absolutely the most bland and uneventful monster species in the series.