If you purchased tickets online for a later time can you go to the earlier time instead permitting there are seats available?

You would have to contact AMC for that answer.

Yes you can go to customer service to do an exchange.

If seats are available i think they would be okay with exchange

They will definitely exchange at box office

I would call them to be sure they probably wouldn't have a problem with that

You have to talk to the location. This is a MAP not a store