As a matter of rule, even the the overall topper is not given a chance to revise preferences after the interview. However, there have been instances (very very few) where people have changed their groups after allocations. A real nephew of Mr.tariq Aziz (friend of Mr.Musharraf) is said to have moved from PSP to DMG many months after allocations.

The PM is the ultimate authority and if someone is able to convince him, he/she can get it changed. And for this, being a topper is not necessary. There have been instances where people have been transferred to police from customs. A lady officer got herself changed from foreign service to income tax etc. So, its the PM who matters, not the merit position.

If your allocations have not been announced yet, you can try contacting FPSC but I think it will be of no use.