As SirWill mentioned in the other post you will need to cancel so it doest auto renue then wait till next month when it runs out then buy patron+ otherwise you will need to spend another 20..
Unfortunately buycraft doesn't provide any way to allow upgrade for subscriptions like it does for one time payments. I also can't think about a workaround which wouldn't allow it to be abused.
Workaround is per-day calculations at the next payment, like all subscription-based service companies do (mobile operators, cable networks, etc).
Unfortunately this isn't in our control, we can only work with what buycraft (the minecraft shop software offers)
