Today I swapped the ECM with a used unit. I successfully read codes and data from the new "used" unit. Which I never could do with my original ECU, a complaint I had for years.

Now all I need to do is change the VIN stored on this unit. I have three cheap OBD readers. Can I do it with these units? Or is there a physical limitation there, so that no one even attempted to make an app that could do this seemingly simple function?

Probably only something a dealership can do.

Just like with home computer, often read and write are two different things. To write, or rewrite as the case may be the scanners need the ability to reflash the chip with new info, something generally only the dealers can do.

with the right equipment, anyone can do this, it does not have to be the dealer. Example; j2534 device and software.