Unfortunately, you cannot override an upgrade. You will need to start a fresh return. Your information may not have changed but the Canada Revenue Agency (CRA) changes the way things are calculated every year which might make a difference in certain deductions or new credits being allowed. When you start a new return, TurboTax will ask you questions about your tax situation and recommend the best product for you. As you work on your taxes, you may have to change products based on the complexity of your return. You may be upgraded to help claim expenses you’re entitled to, but might not know about.
There might be donations in box 46 of your T4 or Employee-paid premiums (medical) in box 85 of your T4 which would require an upgrade to allow you to claim additional credits not included in the free version. Please see the list below enumerating what is or is not included in the free version.