Renewal amounts of lowered donations are causing confusion

[Changaco]

For an automatic renewal, Liberapay's scheduling algorithm usually reuses the amount of the previous payment, even if the donation amount has been modified since then. For example, a donation which was originally €25 per month but has been lowered to €15 per month is still scheduled for a €25 renewal. It's possible to modify the renewal amount, but the donor isn't really guided toward that, so some donors are confused and cancel the renewal instead.