This is rather disconcerting as it happened previously with the Stripe payment gateway. I decided to use PayPal since your support for it seems better.
Basically, I started with a free trial, manually extended it, then signed in as the customer and upgraded the subscription to a higher level plan. The upgrade went well. I then tested a change of term, which also worked. I then downgraded the plan, which also worked. All of this within about 30 minutes.
I had some dinner and came back, and the subscription had canceled itself. The two last log entries for the site in question are (last one comes second):
-- User modifying subscription via PayPal Express: New subscription created (Standard Plan: $24 USD each month), first payment will be made on June 24, 2013 - I-HBM2EYV5USRE
-- User modifying subscription via PayPal Express: Old subscription canceled - I-D479FF8XL8RM
I'm testing in PayPal Sandbox mode but doubt that's an issue -- shouldn't sandbox mode be a mirror image of live mode?
So it looks like the old sub was canceled, but the new sub has yet to be activated...? Clearly this is a problem, since there is an unexplained gap in the customer's ability to access their site.
Can anyone explain why this happened?