You can change your renewal membership at any time:
- Click your name at the top right corner of any page
- Select My Account
- Click on the Billing section near the top of the page
- Click on 'Change/Cancel' next to your membership plan
- Select your new plan
- Click 'Confirm Changes'
Just keep in mind that the change won't apply until the end of your current monthly membership plan. So if you're mid-month, the change you make will take effect when your billing period has ended.