Last I checked, ProSeries won't let you efile an amended return if you didn't prepare the return yourself.

I would just start with a new client file and prepare the return correctly and then add on the 1040X and fill in the first column from the incorrect return.  It is less work and less frustration to do it manually versus inputting the incorrect data into a new client file and then following the steps for amending a return just to have the first column of the 1040X to autofill.