payment gateway handling membership upgrade

I have written a payment gateway for the europayment group.

While I have it working for the initial payment and cancelation. When a member wants to upgrade there membership say from a 30 day recurring to a 90 day recurring, when they click the upgrade button it upgrades their membership but they are not being redirected to make a payment as they would if they were paying for the initial subscription. So they effectively get the upgrade without changing their payment. I have looked at the other gateways and copying what the paypalsolo gateway does, basically has the effect described above. I am guessing I need to register an action or filter, just need some advice as to the right way to go about it.