He hasn't ever switched to the Hollow mode because he wanted to, but when he has been seriously wounded and would be killed otherwise in the imminent future. So I would say it pretty much happens automatically under those circumstances, whether he wanted it or not. So far the hollow within has always bought him enough safe time that he has got the chance to break it and return to his normal shinigami form.
Considering that, if the fight with Kariya wounds him seriously and he's about to die in the next instant, it's extemely likely he will switch to Hollow mode. That should surprise Kariya.