When the ad expires, you can login to your Account Dashboard and click the renew link. The ad will renew, but won't be paid for. Then, click on the Edit Ad Details link either on the payment page or in your account management area. Over in the upper right corner is the upgrade box and just below that is a link to downgrade. Click that and you should be downgraded to a Premium ad. Then pay for that one from your account management area.