Any time you change components it is time to start all over in working up your loads. A change from standard to magnum primers usually calls for an initial reduction of about 10% in your powder charge, then you may carefully and judiciously work upward while monitoring for pressure signs.

About 30 years ago I helped a nephew get started in reloading. One of the issues he experienced was a perception that "magnum" primers were intended for "magnum" cartridges and loads, so he jumped right in with magnum primers for .357 mag loads. No major problems or damage, but still some lessons learned.

Magnum primers can be helpful in achieving proper ignition with heavy loads of slower-burning powders. That doesn't mean magnum primers are required for all such loads. Each combination must be worked up carefully, and any change in components (primers, cases, powders, bullets) must be treated as a "new critter" to be tamed.

Have fun, but do it carefully.