Your actual experience in combining multiple reservations into a single reservation will vary based on the specific skills of the CM assisting you.
The primary issue is how the system works. It expects to make a new reservation / modification for the entire period but there must be open availability in addition to your existing reservation(s).
You want to have your rooms booked in the same category and and ask to be transferred to a Guest Services (GS) CM as they have the ability to handle re-booking situations when a general reservation CM may be unable to make the change.
Note: you don't have to combine multiple reservations just follow the post from
@CarolynFH :
The CM will update your multiple reservations by linking the other confirmation number in each separate reservation. You also should stop by the front desk to verify your reservations indicate that they are linked (noted) on your reservations.
Special note: if you use this method each reservation will still have it's own start date for the 60 day ADR scheduling window and Park Reservation window. If you have multiple package reservations you can just purchase your tickets with the first portion of your stay or the second part based on your park day scheduling.
Dave