I don't think there's a good way to replace the new style bolts with the old style bolts. The new design is simply more robust.

BUT you can make a purely cosmetic fix that will make it look right. Run a standard m5x0.8 tap through the allen wrench hole in the existing bolts. This won't do them any real damage; and then you can put m5x0.8 stainless bolts, with a smooth hex head, into them just for decoration.



The heads may be too thick and may have numbers and letters on them; if so, you'll need to file them down to a suitable thickness and polish them smooth.