Firstly, sorry it's taken me so long to reply here.
@BRiT did prod me that there were questions pending but I haven't had the time to take more than a cursory look, and I didn't want to chime in without knowing exactly what was going on.
I've delved into the user upgrade system and, as I originally configured it, the system won't lock you out from regrading to another tier (either up or down) while you have an active subscription. However, my expectation was that if you did, it'd cancel the one you had and put you onto the new one you wanted, and work out any pro-rated aspect. That's clearly not happening.
So what I'll do is lock out other tiers if you have one active, so that you have to purposely stop the one you currently have to move to a new one. The system doesn't appear to have the ability to let you regrade in a nice way, which sucks, but it is what it is.
As for those of you on multiple tiers, of which I
think there are only two: I'll work out any overlap in subscription payments, refund you what you've also paid at the original tier you were on, and cancel your original subscription so there's just one active. I'll do that now after hitting reply on this post!