Currently I can't find any way to rename the derived files created by a mirror pattern.
The dialog doesn't give any option to change the assigned name.
Renaming them in design assistant results in it being ignored, and the mirror pattern creates new files with the old names on next open and update.
The only option currently is to recreated the mirror pattern, messing up any dependent constraints, etc.
Instead, it should allow one or both of the above suggested workflows.