You cannot modify the dates
This is not true. You can change the dates and retain your shipboard credit and discount. You can do this whether you retain the reservation or transfer it to a TA. What you can't do is cancel a reservation that you booked onboard and have it reinstated to match what you got when you first booked it onboard.
fast43- Rebooking onboard will get anyone/everyone a 10% discount and a 200.00 stateroom credit.
The offer can vary. When I booked onboard awhile ago, the offer was 10% off and $400.00 shipboard credit when booking one of the Antigua or San Juan itineraries.