It will work but to get the most out of Legacy there is a module, one for steam engines and one for diesel engines, to put into the Cab 2 that will make TMCC better enhanced for running on the layout.
The Legacy Instruction manual covers how to ADD the Legacy CAB2 and BASE2 to an existing TMMC layout. It is available for download from Lionel.
You won’t have to readdress your SC2s and Engines, but you will need to Add their assignments to the CAB2, I know of no way to transfer this data from the TMCC system to Legacy, although that may be possible and would most likely be covered in the LIM.