TO downgrade, you have to first complete the Personal Info section, then with the return still open select My Account at the top right, then select the clear and start over option. A few screens later you can select the version you desire to start with.
If you have rental income (as opposed to royalty income) then do not do this, or you will lose all of your depreciation history and it will bite you in the future.
Also note that you can not do this if you have already registered the product, or paid your TurboTax fees.