Primers do make a difference. Magnum primers would raise pressures quite a bit, depending on several factors. While a normal answer would be "go ahead and shoot them", which ain't all wrong, I'd say put those cases loaded with magnum primers away until you have more experience and understanding of primers and pressures. Then you can decide for yourself what you should do with them...

This is a pretty common question on forums and if you googled "can I use magnum primers instead of standard?" you'd get quite a few hits, with reference to several forum threads...