Which payment gateways allow me to modify active subscriptions with the Membership plugin?

I would like the ability to change the amount of a subscription for existing members periodically (an undefined or variable amount of time).

1. Is this possible?

2. If not, are there workarounds (ie. terminate subscription and have them re-enter their payment information for the new amount)?

I currently have 300+ members with 2 statuses (members & visitors). Members pay $15 quarterly for subscription. I would like to increase this amount to $20.

I know there is a means with PayPal to modify active subscriptions.

3. Is this built into the plugin?

4. Do the other gateways support this (ie. 2Checkout or Auth.net)?