Okay... Here are the FINAL Results, after a ton of testing, I finally FIXED IT.
Had to use 3 different programs for it to all come out right. Now, maybe I could have eventually figured out how to do it with less, but I was trying every combo as possible.
The programs I used are MediaMonkey, Mp3tag, and Tag Scanner...
The reason I had to use 3 programs is because each would "do" and "show" you things differently that the other didn't etc.
1. I used MM to "save all" files to put them back to 2.3, figured it might be the better format to use for compatibility. Using MM also caused the Genre tags to not put the double-slashes, but placed semi-colons instead.
Doing this allowed me to use the "remove duplicates" in Mp3tag, because when I was trying to use it before per other instructions in this thread/linked, the Action would end up deleting ALL of my extra Genre Tags, leaving only the first.
2. Selected all files and then with Mp3tag did Actions "remove duplicates".
When I did this, I looked at the files within Tag Scanner and it was showing double dates (when before it wasn't under Tag 2.4), and also, one of the dates it was actually showing the second date for many files having a "full date" instead of just the Year.
BTW, Mp3tag has NEVER shown me the "full date", only the year. So, this is a limitation/flaw with Mp3tag that needs to be fixed. I shouldn't have needed to use Tag Scanner to see the actual field data and extra fields.
Note also that things often displayed differently between the two programs, this is why I had to use both to actually see what was going on. And yes, Extended Tags View in Mp3tag DID NOT show me these things, extra year fields, etc.
3. Then went to MM, cleared the database, rescanned files.
Then used the Date to Year plugin to remove the dates replacing them with the Year instead.
4. Then went to Mp3 Tag ran "remove duplicates" again, and FINALLY, all extra fields have been removed, and the Year field only shows the Year in both Mp3tag and Tag Scanner.
BTW, this all took dozens of "try's" trying the different programs, looking at them at each step, trying different things, etc. etc. to finally figure out a process that actually fixed my tags.
Hope you all enjoyed that roller coaster, I sure didn't...
BTW, I will I think be posting a Bug Report on Mp3Tag and Tag Scanner showing different info.
Another thing they would do for example, is Mp3Tag would just show a single year, and then when I viewed the same files in Tag Scanner, it would show TWO years. And vice/versa would occur.
You can see how incredibly frustrating all this was to figure out. Massive amounts of testing...