An amended 1120S is not filed using Form 1120X, but is done through superseding (or supplanting) the original 1120S return with the revised version. This is why you need to save the previous version as a different file, so that you may keep a data copy of the originally filed return.
The "amended" 1120S is recognized by a small checkbox on page 1 if the box is even checked (which it might not be). If it is not, checked, the new return is designed to replace the previous one (and IRS instructions are to e-file the replacement).
You can see more on this at this website: Superseded Form 1120S
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