Back to an earlier question, TMCC is TMCC and Legacy is Legacy. Swapping in a Legacy R4LC will not make your loco work in legacy. Some parts will, but Motor drive will not. Modern Legacy is DC motor based and has cruise etc...
Also beware R4LC is the hardware, and ERR uses that board R4LC, but with TMCC code. So putting an ERR R4LC into a legacy loco (those that used R4LC; new ones don't) also will not work. More reading on the forum will bring you up to speed. G