I do not believe this is possible. It's hard coded into the BIOS of the system. It would just be easier (and safer) to just replace the HD7 laser. The 400C laser is a lot more unstable than the HD7.
However, the HD7 and 400R type lasers are interchangeable.
Chances are the guy that was going to swap your laser and reprogram your PS2 was gonna do a motherboard swap on you or a complete console swap.
I've seen repair shops swap consoles out, just because they have extra parts. Plus, they would keep your good motherboard that works with the stable HD7.