T-Mobile has not had long-term service contracts in quite a while. You should be able to pay off the phones, get T-Mobile to give you the unlock codes/authorizations, and port your numbers over to AT&T or an AT&T MVNO (in most markets), get new SIMs and continuing using the phones you have. I've never had a T-Mobile contract in all the years that I've been with them, so I'm not the best one to ask about the details.
Does your account have a corporate/organization discount?
OBTW, unless your service is just absolutely unbearable, wait until the last week of your billing period to port out. None of the carriers prorates partial months any more.