T-Mobile is upgrading to LTE as well.. I do not know if the devices can roam.
At one time the hardware had hard code in it that insisted that it only be used on the network that sold it, This is called "Jailing" of a phone or Wi-Fi adapter(jetpack) Older models may still be jailed, It is currently, far as I know (Limited) illegal to Jail these devices so you can not jump networks. but of course many companies care little for the law and do it anyway claiming the device was made before the law went into effect (the law is fairly new, last few years)
To find out if it can be done there are two different procedures.
1: Find out who made the thing and contact them with your model and serial number, Their Tech Support can advise you. This is the method I suggest.
2: Try it and see
NOTE: not all T-moble towers have been upgraded to LTE yet.
