I would plan on leaving AK by 5:30....that way you'll avoid the biggest of the departing crush!!! Then, go directly to the Polynesian. I would bring a tote bag with a change of clothes and leave it in a locker. If you really want to change, then you'll have the stuff right there...go to the Polynesian and change there.
The vast majority of us don't bother with changing clothes unless we are going to a signature restaurant!!! At 'Ohana you don't need to be all 'gussied up'. Just bring fresh shirts with you.
If you go to POR from AK??? You will have to wait for the bus (could be 3 mins, could be 20), then the ride of about 20 mins. Walk to your room, freshen up, change clothes, back to the bus stop...about 40 mins. Ride to MK...about 15 mins. Boat or monorail over to the Polynesian....at least 15 mins, could be 30!!! So, that whole thing will be about 90 mins...which means you have to leave by 5:30 in any case, and you'll be stressed out!!! I would just leave then and go directly to the Polynesian.