I rebooked while onboard and received a $200 credit from DCL. When I got back, I transferred my ressie to Dreams. At the time, they were offering a rebate so I saved $75 off my total, and they were also offering a shipboard credit. I was told I could not get the shipboard credit through them since I already had one from DCL.
Here is the email I received from Dreams:
Disney facilitates the shipboard credit promotion we offer and they have limitations on who can receive the credit. We are not permitted to give the additional shipboard credit to clients transferring their shipboard bookings to us during the promotion. Their rules are that the booking has to be a new one made during the promotion period. Disney policy also prohibits combining shipboard credits. Since your shipboard credit for booking on the ship is coming from Disney as well, they will not combine it with this shipboard credit promotion (which is also funded by Disney).
It looks like only certain shipboard credits can be combined.
Reneé