If you are going to replace a 123gn FMJ with a 150gn SP, then you will need to remove some powder. I wanted to do the same but with an unknown powder that can be dangerous. So, I checked the manual and in almost every case there was a difference of 2-3 gns when you moved up to the 150gn. So, I removed 5gns and worked up from there, same as if I was working up any other load.
My rifles like losing 1.5gns of the original powder and shoot quite accurately. Less than that and I was observing signs of pressure. Now I simply dump out the powder, remeasure, and do up 50 at a time. I started out re-using 150gn bullets from milsurp X54R that I also replaced the bullets in for hunting, then just sighted in with those, shot to get comfortable with the rifle and round, then zeroed with the 20 SP tips, saving 10 or so each year for hunting. When I need more, I have them. Several hundred of the same lots