You'd have to call
DCL and ask them. A Concierge 1 bedroom suite sleeps 5. But, as I said, it's possible that, even with empty beds in the room, you may not be able to add people due to how full the lifeboat for that room is.
Another thing, the deposit for the 2 people already on a Concierge reservation is non-refundable, so that's forfeited.
So, (making some assumptions here), you currently have 2 people in a room that could sleep 5? You cancel one of the two, forfeiting the deposit for that person. The second person now pays single supplement (the room price is equal to two people in the room. But, you add 2 others. In an ideal world, one of the 2 new people takes the already paid for second person slot (but has to pay the difference to what that second person would cost now). And the third person pays full fare for a 3rd person at current rate.
Then the first person on the reservation (one of the original people on the reservation) is a no-show. You'll get the port fees and taxes returned for the no-show.