PayPal Modify Subscription - Payment Standard
I've been looking into this recently. You can create a "Modify Subscription" button to change subscription details such as amount and period. When the user clicks this button, they login and see the old subscription details and the new subscription details, and must approve/save it for it to take effect. One major issue though is you can only increase the subscription amount by 20% within a 10-day period.
To create the modify subscription button just use the regular subscription HTML button with the new parameters you want, and add an extra parameter "modify=2" (2=modify only, 1=modify or sign up).
Also I think that the custom variable field does not change even if you modify the subscription, but I am not 100% sure on that.
After a wee bit of searching, hopefully this will help:
If you became a PayPal Merchant after October 15, 2009, you can edit a buyer’s subscription profile.
As a subscription service provider, you can edit a buyer’s subscription profile in the Edit Profile page. Use this page to update buyer information, adjust the cost or cycles of the subscription, and specify how to handle missing or failed payments.