I can't tell you I downgraded specifically on this phone but I can tell you that downgrading will not hurt your phone. Its only puts the exact rom,kernel etc. Back on the phone that it was shipped with. In the evo forum we would downgrade and upgrade our devices all the time. For some reason or another each radio worked differently with each device. I believe it was a hardware issue.
So in essence you shouldn't have a problem because you are not doing anything to damage the phone. I will contact a dev for you to make sure. His name is xxxvirusxxx on here, well respected dev.
Sent from my MB855 using xda premium