I'm not actually
sure how it works. But, I just tested it with my old v19 Windows license, and I
think your best bet is to get the Master License.
The price for a full copy on Mac is $49.98.
The upgrade from any previous Mac license to v21 is $26.98. But, I couldn't get this upgrade to work with my old Windows license. I think you need a previous Mac license, which you don't have, so you can't do that. Perhaps someone from JRiver could fix this for you if you really wanted to, but...
The upgrade from any previous Mac or Windows license to the v21 Master license is $29.98. This will work with your old v19 Windows license. And, you get one that works on any platform (and it is only a few bucks more).
So, $29.98 is less than $49.98. That seems the way to go for switchers. All the links to do the purchases are in Jim's post here:
https://yabb.jriver.com/interact/index.php?topic=98869.0Make sure to use the trial first for a while.