It looks like there is an upgrade path from Windows only to a master license, because there is a sticky thread in this forum on the URL to do so (this is where I found the URL I had posted after entering my old registration code). The menu item I had posted is performing the incorrect action and needs to be fixed.
For MC22 in my Linux install, however, it is not supporting any of my APE files. This means the majority of my music library is not even playable by MC22 under Linux. I think I will simply stick with Audacious that natively supports my entire library. With this being the case, I will not bother trying to get a master license and simply keep what I really had no plans to purchase anyway (it is 6 US dollars cheaper). When MC23 comes out, I will have already upgraded.