I keep getting subscr_modify ipn notifications in PayPal, though I don't offer any modify buttons on my site. Can a user change the dates or prices of the subscriptions through their PayPal account without going through my site, or is this just caused by things like the user changing their address?
The answer from PayPal support:
In this case the user cannot change the amount or length of the subscription, they can however change the funding source or address associated with their account. Those actions will generate the subscr_modify IPN that you are receiving.