I've looked at paypal but does that support adjusting subscription plans easily?
I generally try to stay away from PayPal, because they're not a bank. Their user agreement states that they're not liable or responsible for "lost" funds, and I've heard of people getting burned by that. If you use them, I would advise withdrawing often.
<snip>"You can let your subscribers change the name, number, regular terms, or currency of an existing subscription without canceling it and re-subscribing by creating a Modify Subscription button."</snip>