I don’t think that it can be done easily with the current subscriptions plugin.
That’s my guess. Maybe you’d create those other plans with the Discourse interface, so it would know about them, and then the webhook could re-connect the user with the new plan.
I’m not sure how hard it would be. A wild guess is 2-5 hours work, the lower end if you just want it to work, and the higher end if you include tests to make it good enough to be submitted as a PM.