Upgrading from Membership to Membership2

This site currently has Membership installed (I would have selected it from the dropdown above, but it's not an option). I have purchased Membership2 (just the single plugin) in order to upgrade but the option is not available. I can either install Membership2 separately or pay to upgrade Membership. Surely I don't have to pay just to update the old plugin before I can upgrade it?

I know this would probably be solved by buying a full membership, but I deliberately didn't do that as I only want one plugin for one site.