I brain control the ai face up defense mode c-crush wyvern. I could not switch it to atk mode so I could atk with it.
Was that C-Crush Wyvern summoned that turn in any way? If so, then that is the problem. The Battle Position of a monster cannot be changed "manually" (by game mechanics rather than a card's effect) by either player if it was summoned that turn.

Also, AI bugs can be inconsistent to the card's script. Something that might work on AI Mode might not work Online, and vice versa. You can always confirm to to be a bug in the card's script if you duel yourself online by using two applications that are open at the same time.