Depends. Are you changing the width of your rim? If you're keeping the same width rim as stock, then the change in offset will be the change in your wheel positioning when mounted (for instance, if your stock wheels have +20 mm offset, and your new wheels are +30, your wheels will move outboard by 10 mm). This is because offset is measured from centerline of the rim.
If you go wider with your rims, distance from wheel face to centerline changes, and this becomes the kind of complicated math that I understand in theory, but always hose up in execution, so I'm going to stop here, and let an engineer or rocket surgeon chime in...