Digging into this more.. our software has problems (a lot), and it's going to cost more money than we can spend to fix it.
So, unfortunately upgrades are going to require a little manual work. I'll create a page for this, but basically:
1. Cancel your current subscription.
2. Sign up with the new subscription you want.
3. We'll refund you the current subscription.
I wish this was easier and didn't expect this to be an issue. The software is supposed to support this natively, but obviously that's not the case.
There is a similar problem with renewals right now because the system isn't tying into Paypal's subscription services properly - those are also going to have to be refunded and reset.
Fun! :)