There are two possibilities that could be forcing you to upgrade.
1. After selecting "I sold a home" it should show two topics to being added on the income card "Principal residence" and "Residential property flipping". Do not click "start" on the second and you should avoid getting pushed to upgrade.
2. If the "Sale of your Principal Residence" wasn't always a principal residence i.e "some of the years owned" was selected instead of "all years owned", then the upgrade is required.
