Putting new SIM into older phone to see if old Sprint service plan is as promised?

Devices do not require activation on T-Mobile.  If your T-Mobile SIM is active and your Sprint device is SIM unlocked.  You should be able to successfully put it in your old Psrint phone to see if it still works.  What is making these devices not compatible though, is their VoLTE and WiFi Calling implementation, which is not supported by T-Mobile.  Devices must be T-Mobile VoLTE compatible to continue to work on T-Mobile's network. 

If your Sprint device is SIM locked, which you can determine by putting the inactivated T-Mobile SIM into the device to see if it tells you that the SIM is unsupported.