Can you change brakes on a 2003 Firebird with a 2003 Grand Am?

There is no such thing as a 2003 firebird, they stopped it in 02

There was one Firebird manufactured in 2003, the 421 SD that never went to production but was revealed at some car shows.

No, F-Body brakes and Grand AM brakes have different disk and caliper sizes made specifically for the handling on their respective chassis. Swapping would not only be difficult it would be dangerous.