"I don't have a before and after scenario to create a MI1040X"

Rule number 1 - never trust the software to keep your only copy of the return ---------- things change and so does the software.  You always need to keep a paper or pdf copy of the return prepared.

Go back into the return and temporarily override the $10,200 exclusion.  Then proceed on with your amending of the return and cancel the override.  It's better than calling your clients to see if they have a copy of the return that you prepared for them.