Follow all these steps BEFORE entering the new w-2s

https://accountants.intuit.com/support/en-us/help-article/amend-tax-return/amend-individual-return-1...

Review the 1040X to see that there is nothing in the middle column and the numbers match the original return. Then enter the new W-2s and you should be good to go. 

I would likely mail the amended return at this time of year. It may actually be processed more quickly than by e-filing next year when the window opens.