Legacy is backwards compatible to TMCC trains. I think your approach is somewhat the problem.
What you need to list is the engines you are considering upgrading (yes, specific model numbers example 6-18XXXX) or details about them like can motor VS pullmor, as this determines what electronics are needed.
But in another honest moment between train fans, upgrading is a costly adventure and has only gone up in cost as electronics supply crunch happens. Does putting $200-$300- maybe even more in electronics into an engine that isn't worth $200 on a good day make any sense? I'm not saying this to downplay your collection or bash upgrades, there is a time and place, It is a fun learning experience, but again, the almighty dollar does come into play. Sometimes, it much more viable to sell off a valuable conventional engine in stock form and buy the modern used TMCC equipped equivalent for less than it would take to upgrade a conventional engine adding TMCC and sounds. Not to mention, can motors perform much better at slower speeds and so again, depending on what you have and were intending to upgrade the modern already command model might be night and day improved VS throwing a bunch of money into an olde model.
Another factor that comes into play- you could just get a Legacy Powermaster and vary the voltage on the track and run conventional trains via remote the way and since you are only doing the track, nothing to the engines potentially saving you $$. Again, one and done vs modifying each engine.
