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. If you purchased hours, you will have access to those hours for 28 days from the charge date. Unused minutes do not rollover.