Not even JHP bullets are the same.
I did a chrono test earlier this week where I tested two different JHP's with the same amount of powder, 5.5gr of Longshot:
The first bullet was the RMR 124gr expanding JHP. The length of this bullet is about .581", and I loaded it to a COL of 1.100". The bullet diameter is .3555".
9mm, 3.9" barrel
COL: 1.100""
RMR, 124gr, JHP, Longshot, 5.5gr, CCI500
Average: 1085
ES: 25
SD: 10.1
Force: 324
PF: 134
Velocities: 1093, 1071, 1088, 1096, 1080
The second bullet I used was the MG 125gr JHP. This bullet is marketed as a 357sig, bullet but can also be loaded in 9mm. The length of the bullet is about .550", which is .031" shorter than the RMR bullet. The bullet diameter is .355". The COL I loaded this bullet to was 1.070", which is .030" shorter than what the RMR was loaded, but should result with about the same seating depth or available powder capacity in the case.
9mm, 3.9" barrel
COL: 1.070""
MG, 125gr, JHP, Longshot, 5.5gr, CCI500
Average: 1096
ES: 27
SD: 10.8
Force: 333
PF: 137
Velocities: 1093, 1093, 1093, 1116, 1089
As you can see from the number above that two different bullets using different COL's performed about the same, because I adjusted my COL based on the length of the bullet. If I loaded the MG to the same COL as the RMR, then I assume the velocity of the MG would come down since there would have been less pressure.
I think Dudedog posted some chrono data once showing how the velocity change (pressure) based on a difference in COL for the same bullet. I will see if I can find it and add it to my response.
Decide on a COL, make some calculations, and then if the your bullet will be seated lower in the case than the HAP, then start with a lower powder charge. If your bullet will be seated in the case a bit higher, then you should be able to start at the same MIN powder, or maybe a bit higher.
Good luck and be safe.