The simpliest way to convert a PSP from a heel release to a P7 button release would be to sell the PSP and put the money towards a P7.
To mechanically change out the release would be a custom job. Considering how hard it is to get HK parts, especially P7 parts, and what a good gunsmith would charge, this would get very expensive, very fast.
Btw, I've owned both a PSP and a P7M8 and I actually prefered the PSP release. I had problems with the P7M8dumping the mag when the release was bumped in the holster.
I later realized the whole P7 series was not for me and sold my PSP and my P7's. I'm happy I tried them out and I'm happier with other guns.