PayPal Modify Subscription - Payment Standard

Question

So I've implemented PayPal fully into my system. The system works on user 'packages', all at set prices. Each user has a subscr_id (set by PayPal) assigned to them, along with details of the package they've bought.

I need to be able to modify the subscription so the user can select a different package. Essentially I just need to be able to change the amount and the 'custom' field (this contains package details, coupons used, etc).

I would have thought, and hoped, that it would be as simple as showing the user another form with hidden fields for 'modify', 'subscr_id', 'custom' (new value) and 'amount' (new value). Unfortunately I can't find this anywhere online, and PayPal's documentation is so monolithic that I find myself going round in circles for hours at a time.

My IPN is all set up and working fine, I literally just need to build a form that allows me to set the 'amount' and 'custom' variables to something new... the bit I thought would be easy! Any help is greatly appreciated. Thanks in advance.