larchitect Sorry for delayed reply. Actually, I replied earlier though forgot to click the button to reply 😅
Currently, there is no way to "upgrade" to Indie. In general, Single and Indie are two different licenses, Single aims for a regular work desk, while Indie aims for freelancers with two PCs as Indie according to EULA cannot be run at ONCE on two different PCs. It highly depends on POV as an "upgrade" from unlimited revenue to 100K yearly Indie limit cannot be considered as an upgrade for obvious reasons, though the ability to run RN on two different machines in two different places at different moments by the same person can. As you see it's not so obvious and the internal decision was to keep it nonupgradable.
Maybe this decision should be revised though IMHO this is a kinda "apple and oranges" case. You can always try to convince me 🙃