Can I purchase turbotax at Amazon, get a code and complete my taxes w/o having to start over? Ihave been a Turbotax customer for over 10 years.

Please follow the FAQ here if you are purchasing TurboTax from another retailer.

Are you asking about switching from the online version to the Desktop program?  Yes you can do that.

If you already started in the Online version you can move your return to the Desktop program even without paying for Online.  If you already paid for the Online version you can contact them to give you the program download.  

First download the .tax file from the Online version,  

https://ttlc.intuit.com/community/saving/help/how-do-i-save-my-turbotax-online-return-as-a-tax-data-file/00/25740

Then see, How to Switch to Desktop,

https://ttlc.intuit.com/community/choosing-a-product/help/how-do-i-switch-from-turbotax-online-to-the-turbotax-software/00/

After you get the program installed the first thing to do before you open your tax return is to update the program and install any state programs you had.  So you first might need to start a fake return to be able to download the state program (go to FILE - New)

Then you just OPEN the .tax2020 file, go up to File-Open.  You don't import or transfer it.

Or we can tell you how to transfer from 2019 online into the 2020 Desktop program.