I have never had this happen before or seen it. I fired my HC and promoted my OC, basically they switched roles, WHAT???

Promoted OC - https://afl.myfootballnow.com/coach/844
Demoted HC - https://afl.myfootballnow.com/coach/400

Well, you have to have an OC. The AI recruited the best qualified available candidate for you ... If you had done it on purpose, you'd get props for a smooth move. :)

Surely the position should be vacant until you hire a new one , no?

I think it only stays vacant up until TC, any time after that the AI will take over. If I'm wrong about that, somebody please correct me.

This was just before the Championship game, its now the eve of End of season, as far as I'm aware its down to the owner to replace any coach
https://afl.myfootballnow.com/

Will the game engine even let you not have a coach at that time? I bet the fired coach was re-assigned to fill the vacancy.

Must be the case, I've just never experienced that before

https://moguls.myfootballnow.com/community/6/5298?page=1#32101

Correct, every game stage must have a full coaching staff even if you are not playing, so the AI will find the best available coach to fill the vacancy.

Thanks for clearing that up