When you are using a fast powder in most pistol loads you won't see a great difference using a magnum primer in loads under max levels. There will be a difference so you really do need to work the load back up just as you did originally. You will see greater differences using slower powders. It seems to be higher pressure differences the slower the power is in pistol loads.
Yes, it can be done. Some care to watch for pressure signs and working the load back up should be done. In larger cases than 9 mm such as .357 magnum I prefer a mag primer even when it is not needed for the powder I'm using.