Currently have a iPhone 5s with AT&T so what would happen when I switch to Tmobile?

I don't plan on getting a new phone the same phone if I have to get a new phone it would still be the iPhone 5s.

You should be able to keep the same one if you ask ATT to unlock it. If ATT cancelled your service for non-payment or you broke the contract early however they probably won't unlock it.

You would have to call ATT and get it unlocked but they are going to be reluctant to unlock it so call them and tell them you are going out of the country for a week and you need your phone unlocked. They will unlock it within a day.