My TA said the same thing. While we were on the Magic over Thanksgiving,
our head waiter said the first of the new ships would be sailing by the end of 2010. I considered then I might change my Oct 2010 date to the new ship.
He added that they would be moving 30% of staff from the Magic and 30% from the Wonder to the new ship. He said service would suffer on the older ships if they took more than that.
If you rebook onboard and later change to a different date, you pay whatever the going rate is for the new date minus your 10% discount. Either way you haven't paid in full, just the deposit which would transfer along with your new date.
If you decide to cancel completely, you get your deposit back but then lose any onboard perks if you decide to call back and book again. I would never completely cancel a cruise that was booked onboard. You can always change the dates 50 times if you like. Of course,
DCL is holding your deposit all the while.