After you file

It would be easier but it is not how it is done ... the W-2c didn't change box 1 so you can't just do so. You must only amend the 2015 return for the W-2 changes and then do the "right of claim" on the 2017 return.  Good news ... since you must be using the downloaded version to amend the 2015 return you can make a "test" copy of the return and use the copy to make the box 1 correction and see the tax difference to use on the 2017 return. 


If you want to play around with different figures and tax scenarios without affecting your original return you can

>>>In the TurboTax CD/Download software by creating a test copy:

1.  Open your return in TurboTax.

2.  From the File menu, choose Save As.

3.  Give the copy a new name to distinguish it from the original (for example, by adding "Test" or "Example" to the file name).

4. Click  Save. You are now safely working in the test copy and anything you do here will not affect the original.

 https://ttlc.intuit.com/questions/1900642-how-to-make-a-test-copy-of-your-return


Turbotax on-line does not support the "claim of right" computation. Turbotax desktop, in the forms mode, permits the credit to be entered in the "smart worksheet" above line 72, but you still would need to compute it manually.