So here's my thoughts. Your Carbon is working really well for you in the current state of the tank, you've just noticed your PO4 slowly getting too high. I would water change out of the high PO4, then run like 5-1 Carbon to GFO to start. Once you've got your fish in there and are feeding regularly and dealing with all the pooh and whatnot associated with that you'll have a better idea of how your maintenance schedule is working for PO4 and NO3 balance.
I wouldn't go and start making major changes to things right before you're about to put your fish back in as things can spike back out of control very quickly then it's much harder to get them back in line.