Thanks for the comments.
I went to the link that Joe supplied (thanks Joe) and it listed 2 enhancements and NO fixes for 4.20:
1) Adds support for larger engine board memory sizes (4MB or larger). The engines that use these new boards are the PS3.0 HO Diesels and the PS3.0 Steam/Diesels
2) When loading either a PS3.0 HO Steam/Diesel or an O gauge/G scale PS3.0 Steam/Diesel engine you MUST have DCS 4.20 software installed in your TIU (and remote)
3) Under the category of "Version 4.20 Fixes" it had 'NONE'.
I now only have 2 MTH engines: both are tin-plate (by Lionel) PS3 steamers. Comment #1 makes it sound like there are 4.20 features that require a bigger board but I don't know how to interpret that comment since no enhancements are listed. Comment #2 kind of bothers me, unless by 'loading' they do NOT mean ADDing it.
I took RJR's advice too: Everything is currently stored away but I dug out my TIU and both engines. I used the 4.2 new remote. I set up about 5' of track and tied in my TIU to it. Using the new remote I was able to:
1) Successfully add both engines to the new remote.
2) Successfully alter engine controls such as MAX speed, volume, Smoke Volume, etc.
3) Successfully START both engines
4) Successfully ran both engines forward and reverse.
5) Successfully ACTIVATE things like Whistle, Horn and Smoke.
6) Successfully do a SHUT DOWN.
Now that I typed that, one thing that I should have done but didn't think of at the time: make sure that both engines still fully functioned with the 2 older, 4.10, remotes.
I don't know what to think at this point. It SEEMS like leaving the remote with 4.20 is OK. I wish I understood the comments more fully though, like what does it mean "loading an engine"?????
As Always - thanks - walt