If you downgrade from a Pro account to a free account, you will retain access to the most recent 3 codes you made. All other codes left in your account will automatically redirect to a page specifying the code was deleted by its Creator.
You can change your plan at any time by clicking on your name in the top right corner of the page and selecting Account. Next, head to the Billing tab to cancel your Pro plan.
Should you wish to reactivate your codes that were deleted due to the cancellation, all you have to do is sign back up for Pro!