To quote from NRA's "Handloading" book, "A satisfactory rule of thumb is to decrease the charge weight in the heavier cases by 12 percent of the difference in case weights."

Doing so should ensure that pressure in the heavier cases is not greater than was experienced in the lighter cases with the original charge, although velocity may differ.

A somewhat more scientific approach is adjusting the charge to maintain the same chronographed velocity. Resulting pressure should be close enough for safety unless there's an abnormally large difference in case weights (more than ~20%).