Probably should have this in the reloading section.
That said Alliant has 6.7 max listed for a 1.125 COAL 9mm 115gr GHDP. Based on what I found on the net a GDHP is .525 OAL. To keep the case volume the same measure the length of your FMJs and see how much longer or shorter they are. If they're longer put them out of the case that much more or if they're shorter that much further in the case. This will let you keep the same internal volume in the case so you don't get excessive pressure. Reduce your load 10% to start and work your way up looking for pressure. 9MM is a small volume case so be careful loading it up.