Changing to magnum primers can raise or lower pressure or do neither. It's a bit complicated. What distinguishes a magnum primer from a standard one is that it makes more gas, thereby raising the start pressure for the powder burn, something particularly important for slow spherical propellants with their high deterrent coating concentrations that make them extinguish easily if pressure is inadequate. In some pistol cartridges in particular, the greater gas quantity can unseat a bullet before the powder burns much, thus increasing the volume the powder burns in, which lowers peak pressure. In others it raises the pressure. There's just no predicting what your brand will do in your chambering with your powder and bullet combination until you try.
As a general rule of thumb, I've never seen a primer raise pressure higher than what about 5% increase in powder charge would do. So, use a chronograph to measure your current load's average velocity. Then drop the charge 5% but use the magnum primers, and see if the velocity goes up or down. Adjust the charge until the average velocity is a match.
One thing that can happen when you are shooting a load that doesn't fill the case well, is a magnum primer can actually improve your velocity consistency. You might want to watch out for that, as well. If it gets worse, however, that's a sign the primer is unseating the bullets.
