There is no mention specifically of who made the change but after doing a ton of reading I have heard all these people sighted as doing the name change:
2nd Doshu Ueshiba
and random other less likely candidates. One thing is for certain no one seems to have a lock tight answer either way (maybe there is no lock tight answer for who or when...)
If I had to put weight upon any one person's words in regards to this, I would definitely do so with Peter Goldsbury. To quote:
Peter A Goldsbury
However, I can see why Kisshomaru Ueshiba decided to change the prewar counters completely. 一教 (ikkyou) etc represented a major break with prewar tradition.
Peter's post goes a long way to indicate that Kisshomaru changed the names. But, I would send him an email or PM and ask if he had more information. I don't know if he's still in China, but I'm sure he'll get your message.
Remember, also, at this time (after the war), there was only a few people around. Ueshiba was in Iwama doing his own thing. Kisshomaru was left in charge of things in Tokyo. He had a few people around, including Tohei, but for the most part, aikido wasn't all that popular until later. Can't recall off hand, but probably not until mid to late 1950s.
Remember, too, that at about these timeframes, when Kisshomaru was trying to get Aikido popular, that both Shioda and Tomiki were also teaching "aikido". At least one article I've read directly shows the rivalry between Kisshomaru and Tomiki.
Toss in that Mochizuki is quoted as saying Kisshomaru and Tohei changed the names. It isn't a far stretch to conclude that Kisshomaru changed the names. He was, after all, the person in charge. Even on the off chance that Tohei suggested the change, Kisshomaru was the final decider.