What type of base grip are you replacing it with?
European size 2 - (4 1/4) and 3 is (4 3/8) correct?
I'd never recommend going larger but going smaller is fine. Replacement grips vary in thickness and most overgrips are abooout 1/16th inch thick which is a little more or less than 1/2 a size depending on how you stretch it when applying (eh, to my knowledge at least). The only concern you might have is that different grip sizes can slightly
alter the balance and swingweight of your racket and for some, smaller grips cause them to spin the ball more. Apart from that, I think it's fine. Unless it's significantly cheaper at that store, you might consider returning it and finding the same grip size elsewhere, though.
Edit: Ok, I just found the tennis-warehouse "Increasing Grip Size" page that might help you out a bit