It sounded to me like OP just wants to change tempo of midi, not touching the audio. The difficulty then is to find the correct tempo as recorded already.
In S4 at least, I often changed tempo of part of song and did that without any stretching of audio, but regretted it did not have a lock of clip start to the tempo map. I had to move audio clips manually. And of course rerecord audio on the part with changed tempo.
Have not been doing this in S8.5 yet, but there must be a default streching of the audio then.
You must be able to turn this automatic audio stretch off is my assumption?
If not finding the disable of this stretching, I would just start another project with tempo set to what was the case you wanted(run audiosnap to find out), and then import to audio tracks from the old project. As waves import separately.
My guess is that it would work. Sonar cannot assume anything about tempo in wave files.
But there must be a disable of functionality, I'm sure of it and even remember reading about some keyboard shortcut for doing it.
I would search help which is very good in Sonar...