Regardless of whether you top it off now or not, you will lose some to racking losses, you simply can't and don't want to rack everything over. I am not experienced with co2 charging, so I am not sure, but I am sure that there are those that support both trains of thought. I personally would rack over and top off with an apple juice and honey mixture equal to your starting SG. But yes it will continue to ferment, but I don't mind this. It won't create much lees, but if you have a problem with that then don't do it. Your other option, racking to a 3 and 1 gal carboys is the simplest. You won't lose much if any, if you are starting with 4.5 gals. Once you count in the losses due to racking, you should be able to top off both carboys perfectly for aging.