Today when I clicked on Adblock it was showing "Upgrade", even though I bought Premium two days ago.
I went through the process (had to go to help screen... then the upgrade screen... the scroll a fucking long way to the bottom) to receive an email that somehow activated it.
For starters put the "Paid for Premium before?" link at the TOP of the upgrade page, so we don't have to search so hard for it (cause I'm betting this is going to happen a LOT).
Secondly, add something to the plugin so that if it does forget, we can reactivate from the plugin, rather than going through the upgrade page B.S. Clicking upgrade is not an obvious solution to a problem that shouldn't happen.