The single "gotcha" with upgrading to Legacy from the old TMCC system is the old PowerMaster units. They receive the 27mhz signal direct from the CAB1 and not from the serial data or the 455khz track signal. That being the case, in order to use them with Legacy, you need a Legacy PowerMaster Bridge.
There's nothing that you can do with the TMCC system to a locomotive that you can't accomplish with the Legacy system.