So currently I have 325 high profile implants and was thinking about exchanging for 425 moderate plus implant?

Exchanging 325 cc high profile implants for 425 cc moderate plus implants could be problematic.  By not adding enough volume to later feel the surgery was worth it, if you are interested in increasing size.  Understand that this would also give a significantly wider implant.  Usually when going to a larger size there is a transition to a more narrow type of implant, not a wider type of implant.  That is not to say it is always inappropriate.  If your chest is somewhat wide and the 325 cc high profile implants were too narrow, then the increased width will make for a nice enhancement and better proportions.  Most women desiring larger implants should add 150 to 200 cc or a little more to be happy with the change in size.  I hope you find this information helpful.  

It depends on what size you want to be?  Depending on your height and frame, 100cc increase in size should make you go up a cup size. You should show your plastic surgeon photos of how large you want to be and let him figure out what size to go up to.