I have twice stopped FFLs from using the assy number on the yoke cutout for the serial number on pre model guns. I thought about letting them go ahead, but, if I ever had to prove where I got the gun I wanted it right.
I would bet it would be hard to find a gunsmith willing to do that. What I am sort of HOPING for would be some letter from BATF that said it would be OK to re-stamp the numbers.