I'm always hesitant to opine on this as whenever I've tried this I get a fare recalculation or additional costs that makes it not economical though in principle it ought to be possible for a change fee. The best way to find out is to call.

Don't get phased by the switch of terminals by the way, the shuttle is very easy and nearer to international arrivals south than up to south security, you head towards the station and sharp left down a travellator to the platform. E-Gates are usually fast (but usual advice is to walk briskly off a long haul), and the main risk on a late arrival is baggage arrival, but again Gatwick is OK for this. You arrive at the North Terminal at the U2 bag drop level, in front of you to the left, and it's really not too bad. It's not a leisurely transfer but it should be fine. U2 do offer a flexible on the day ticket supplement I think but I assume too late for that.