I strongly suspect they will move to CDMA, as IIRC Sprint owns far more towers than t-mobile. On the other hand for LTE they should be compatible.

[edit] As I said in another thread, my information for that is a few years old, so this may not be true anymore.

Sprint has more towers, but they effectively run three different networks on top of them, making it a terrible fragmented mess.

More likely that they slowly decommission the CDMA network as both networks move towards being 100% LTE.

I miss Wimax.