I would suggest just making a backup of your store and then cancelling your Unlimited Site Plan. Then, when your Unlimited Site Plan subscription expires (in a few weeks) purchase the 1-site plan again.
Then continue with the 1-site plan. If something terrible happens and you need to restore to your backup, then you can upgrade to the Unlimited Site Plan again for another month. Otherwise, if nothing happens, you only pay for the Unlimited Site Plan 1 time (not every month).