You didn't say if you were shooting 148 grain wadcutters seated flush with the case mouth or some other bullet seated further out. The difference it makes is that the former doesn't leave a lot of empty space in the case where the latter would. In the latter instance you may actually find you get better accuracy from magnum primers just because the do a better job of pressurizing all the empty space. A magnum primer behind a 158 grain round nose bullet seated to the max .38 Special COL is probably making lower pressures than standard primer behind the deeper seated 148 grain LWC is.
Broadly speaking, though, the advice to reduce charge is good. In .223 I've seen magnum primers make the equivalent pressure difference of about a 4% increase in powder charge, so I only drop rifle loads 5% when making the small rifle primer switch. The smaller volume of the handgun, though, probably makes the 10% reduction more reasonable to do. If it were a rifle cartridge I'd then say to roll six rounds that go up in 2% steps, but that's too small for most scales to weight in your case. So I like the idea of just dropping the load half a grain, then going back up in 0.1 grain increments to see if you can tell the difference.
In the end it may make no difference at all. The primer may just unseat the bullet sooner, leaving the net peak pressure largely unchanged because the powder starts building pressure in a larger volume. Can't know until you try, though, and that's why the reduction and build up is a good plan.
As also suggested, a chronograph would make this a lot easier.
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