There exists no such thing as an 1120S-X. If you look at the 1120S, the "amended" box (Item H (4)) at the top of page 1 will be checked.

There are a couple of places to make entries explaining what changes are being made on the amended return. My recollection is that you make one set of entries to generate the notation that prints, and a second (mostly identical) set of entries to generate the amended e-file.