@Jonathan , it appears it's currently not possible to upgrade accounts. At the end of the process a notice is given that the account isn't able to process live transactions.
I suspect you'd like users to be able to do upgrades. Mine expired today, and the ads are annoying, but I can't renew it.
My membership says it has expired, and I'm getting ads now, except the upgrade can't be processed. It says this account isn't authorized to process live transactions.