Successfull in what?
I just (last friday) went from 7.1.6.0 stable, with 3.0.2.2, back to 6.1.22, then 7.2.9, installed new TWRP 3.1.1, then 7.9.21 all without problems.
Funny thing is, that bootloader never got locked. Tho I have been at android 7.0 before, but battery was so bad I went back...