You can upgrade from GPL to Pro by going into System Settings -> Upgrade to Pro.
When upgrading, all settings will be retained. When downgrading, most settings are retained, though things like resellers, or other Pro specific settings, will no longer be available.
You can use Paypal, though our shopping cart doesn’t support it, we’d need to process those manually. Due to that, we can only do Paypal licenses a year at a time.
And lastly – reverting to GPL isn’t as simple as upgrading, there’s some manual steps involved, but it’s not that bad… you can see details on how to do so here:
If you’re interested in proceeding, go ahead and open a support request in the support tracker for that (using the “Support” link at the top of the page, and we can continue there. Thanks!