Let me go look on the upgrade forum and see if it is self evident to go to at least MC 22.
MC 24 is the latest version and is the only version available that you can upgrade to but it should support the Pono Player.
That said, I have to agree with Roderick though, Pono is more-or-less dead. If you buy one, you likely won't get any support from Pono for the Pono Player if something goes wrong with it. There are alternatives to the Pono Player out there though, like players from FiiO and Astell & Kern.