It's doable, Maestro did it, but it's much more intensive than just body panels. There are underlying supports and brackets that need to be swapped as well.But you can swap the entire front end of an 06-08 with an 03-05 correct? Lights, surrounds, grill, bumper, etc.
Do you have any pictures of this? Are the points where the clips that mount the grill to the car made of aluminum as well? I broke some of the plastic bits and a friend has a beat up 03 forester that would trade grills.There is also a slight difference between the 03 and 04/05 grills. They are interchangeable, but the 03 grills are a single aluminum piece, where as the 04/05 Grills are molded ABS with a detachable chrome trim ring.
When I blacked out an 03 grill a few years ago, I sanded it down and you can actually sand off the stock gunmetal paint and polish the grill up real nice if that was your thing.