Sleeper,
First you have to determine... What control do you want? TMCC or DCS?
As mentioned above the ERR products are a great fit for TMCC. GunrunnerJohn and others have various threads on their upgrades. As a mater of fact, here is a thread on an ongoing upgrade.
https://ogrforum.com/t...he-upgrade-has-begun
If you go the DCS route, you need to buy an upgrade kit or obtain an inexpensive donor engine and rob the parts out of that for the train you want upgraded. I'm doing that now for some RMT BEEPS.
https://ogrforum.com/topic/ps3-upgrade-questions?reply=39441158242690081#39441158242690081
GGG here on the forum is also in the business of doing these upgrades or folks who don't want to tackle them themselves.
Good Luck.
Ron