You can't just switch, and this will not work if you have already registered or paid your turbotax fees.
With your return open so that you can edit it, you must at least complete the information for the primary tax filer under the Personal Info tab. The click My Account at the top right and select the clear and start over option. A few screens later you can select the flavor of TurboTax Online you desire to start with.
Yes, you will lose anything and everything you've already entered, and there is absolutely no way possible to avoid that. So in a nutshell, your choices are to clear and start over, or continue on with what you already have.