Yes, to downgrade you would need to clear and start over. As long as you haven't submitted payment, deducted the TurboTax fee from your refund, or registered, you can erase your return and start from scratch in TurboTax Online. Please see the FAQ link provided below for more information:

Can I downgrade to a lower-priced version of TurboTax Online?

If you have registered or it won't let you clear to start over you can just abandon this account and start over with a new account.  How to start another return in the Online version

https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-start-another-return-in-turbotax-onli...