The JRiver licencing is very generous, but I have never seen Jim allow an old licence, or any Windows licence, to be converted to a Mac licence. Why convert an older version anyway? Old software licences don't hold any value once a newer licence has been purchased. There is no trade-in value to the vendor (I've been there, done that with customers who tried to trade in a licence they bought). So if you aren't going to use the Windows MC23 licence anymore, that would be the one to trade.But I don't think that will happen. Besides, an upgrade from a Windows licence to a Master licence isn't that expensive, even though you have missed the discount period.
If you are possibly going to run MC on a Mac, but maybe run it on a Windows PC as a Client at some stage, you really need to be using the same major version on both platforms. Plus of course, you could run it on Linux as well.
See
https://wiki.jriver.com/index.php/Upgrade_to_MC23_Master_License That page has some info and a direct link to the upgrade page, where you should enter the Registration code, and then you'll find out what the upgrade will cost. I think it will be a maximum of
$36.98, but I'm not JRiver, so try and see what is offered.