Members Plugin - PayPal "amount can only increase by 20%"

I have a site which is going to allow several different membership packages, each going up by over 20% in value.

Currently if I click "Upgrade" on one of them and go through the steps on PayPal it has an error "Amount can only increase by 20%".

What can be done about this? Ideally if the user will not be able to upgrade because it is too expensive then the option should be removed, and there should be an easy process for them to cancel their current subscription immediately and choose a new one, however even cancelling a subscription and trying to set up a more expensive one triggers the error.

Upselling to a higher subscription is an important part of our business strategy, what are we to do about this?

Shaun