It was answered in that I did it and saw what happened.
To recap - The amended return does not void the originally filed return or serve as a subsequent joint return filing. What it does is cause an IRS adjustment to the original return. For the non-filing spouse, the original separate return becomes the originally filed separate return for both spouses! That may not be what you want, particularly if you have to later produce a tax return transcript to verify your income; for the non-filing spouse, the tax transcript will show the filing spouse's income only. While an account transcript will cryptically show adjustments in tax and filing status, the details of where those adjustments came from (the Form 1040-x) are not maintained by IRS.