Mike, you will need both a new Railsounds board and an R2LC for full TMCC. I guess when Lionel said upgradable they meant its just a simple board swap, which it is. You only need the audio board, not the audio power board which is already in that engine.
They used to sell a kit with these two boards, and may still but today you have the option of many more audio boards to choose from. When shopping for R2LCs get one that has the 08 code. That one is compatible with the most engine setups.
Generic steam
Or use anyone if specific road name and engine number is not important.
https://www.lionelsupport.com/...p;q=RS4E&match=0
Pete