When PS updates and corrects a calculation within the program, it automatically revises affected returns; filed or not. If filed, they’ll create a 1040x template which you can use and file.

Locking the return may protect what you’ve filed but as soon as you unlock and calculate the new revision(s) will be applied. 

suggestions:

1. Always save a pdf copy of the return you filed. 
2. I find the locking mechanism cumbersome… at the end of tax season I change the properties of all my PS client files to read only. It effectively locks all the returns. If i have to amend, then I’ll change/ revise the properties for that client. 
3. Somewhere in the program, there’s a listing of returns that have been changed after filing… i think there’s also a notification when they do change a return but don’t remember where…