You have to look at the piston sizes, and do some calculations to figure out if you can use the 2003 Boxster rears upfront.
For my German Look, I'm running Boxster calipers on all four corners, fronts on the front. Rear calipers on the rear. The fronts use 944 hubs modified to my beetle spindles, and 1976 911 rotors. This mates up perfectly with the front calipers (with custom caliper mounts). On the rear I'm using 944 bearing retainers (which have the caliper mount built in), 944 turbo rotors, and the Boxster calipers. No mounts or adapters needed.
The piston sizes come into play with the Rally Bug. Currently it runs the 2-piston Ghia calipers (solid rotors) on all four corners. I have a set of 951 rear calipers coming off an Audi that I thought would make a great front brake update for the Rally Bug.
BUT the combined piston area of the four piston Porsche rear caliper is
smaller than the combined piston area of a 2-piston Ghia caliper. Thus, the car would end up rear biased. Can't use em.
So, for your rear boxster calipers on the front question, you need to do some piston math and compare them with your rear calipers. I use
https://brakepower.com/piston-area-calculator.htm
-Dave