Does anyone know can Bitmain tell if firmware was changed in the past if i revert it to original one?
AFAIK, no.
I had a machine which pooped 2 hashboards (on original BITMAIN firmware) and I tried custom one just to see if voltage change or frequency change would bring it alive (worked fine for some time), and then I decided to RMA it. Just returned to original firmware and they repaired it w/o questions.
It was in warranty.