This is going to be a mess. Two terrible networks together equals ???
Where will the mess be? When the dust settles there will be only 1 network, Period.. As this will be simpler than expected compared to the sprint/nextel fiasco. With that sour deal they tried to keep and maintain both networks, that was a fail. The details have been given with TMO-Metro that there looking to shut down Metros cdma network and moving people to TMO's GSM/HSPA/LTE Network.
Seeing as how most of the areas that Metro is servicing TMO currently has coverage its a simple phone swap out at the easy point. The only real issue is re-farming Metros spectrum to be used with TMO service and deciding what Metro towers to keep that overlie or closely overlie a TMO tower.