I feel lame for asking this, but after looking for a long, long time I can't seem to figure out how to have a TurboTax person review my 2020 return prior to filing it.
No. You can only go one way from online to Desktop.
If you want the LIVE expert help and review you need to use the Online version. You can't apply the Desktop CD/Download purchase to Online. You have to pay for Online separately. You will have to get a refund for the Desktop program. Then you can use the Online web version. You will have to start your return over in the Online version.