Apologies for not getting back to you sooner. I put out some questions to two of my Sensei's back in Japan, one seventh dan who used to study with Saotome Sensei many years ago, the other a 6th dan who was a student of Yamaguchi Sensei. It took a while to get the answers back.
The first one said that oyo henka is where uke changes the technique to something else because that's what the situation requires (e.g. uke stops the initial technique). This can be a variation of the initial technique (perhaps where I was originally coming from) or it can be a totally different technique. He said that kumite was not an Aikido term, there is no kumite in Aikido.
My other sensei did not understand either of the terms, he said that they were not terms that he had used or come across in Aikido.
I think we're pretty much agreed on our understanding of oyo henka
, but haven't been able to clarify kumite keiko any better