For many years I've used turbo tax down load. I'd like to use turbotax live for my 2017 taxes because of a large charitable deduction. Is this possible?

[Critter]

Yes you can but why is a mystery ?   The Downloaded version is superior and cheaper since you will need at least the Online Deluxe version. The charitable donation has nothing to do with the choice of program you use. 

To transfer the tax file from 2016 Downloaded program to the 2017 Online program 

Last year's tax data file (.tax files) can be transferred to Online Deluxe and higher-tier versions.

1.     Sign in to TurboTax Online, if you haven't already done so.

2.     Select Tools from the lower left  corner.

3.     In the pop-up window, select Transfer last year's TurboTax return from your computer.

4.     Read the warning and then click Continue (or Cancel to abort the transfer).

5.     Browse to where you saved your .tax2016  file and select it.

6.     Click Transfer Return and wait for your file to finish uploading.

7.     When you see Transfer Complete, click Continue to start working on your return.

https://ttlc.intuit.com/questions/1900883-transfer-last-year-s-return-into-turbotax-online