Resort Reservation Modification vs Cancel and New Booking?

Original trip dates: April 14-21
Current trip dates: June 9-14
This trip is just DS and I to celebrate his 20th birthday (tomorrow). We have already pushed the trip until June and the reservation number stayed the same and we got free dining and everything seems great. The original plan was that we would book a bounce back while there for the whole family to do a December trip. Well, now I’m not too confident our June trip will happen and there isn’t much opportunity for he and I to go later in the summer/fall—particularly since the December trip involves extended family members and I don’t want to do trips too close together. Hypothetically, could I modify this trip again for December and add additional family members or will I need to cancel and rebook? I know cost will be added for additional family members, but I’d rather just have Disney keep my money instead of refunding and having to go through the whole process again.