Sorry I'm not sure about modifying a previous reservation to include a Bounceback offer, so I'll bump your post back up to the top for you and hopefully someone who knows about that will answer
I do think that any resort guest can book a BB offer before checking out (if a BB is offered during their stay). You would have to each deposit separately of course, and choose from the dates and resorts on the BB offer.
However, I wanted to mention if you are going in September, that last year the resort guests staying from 8/31-10/12/07 received a
Free Dining BB, NOT a room discount BB. They were able to book Free Dining again for the next year (a year in advance) ONLY. There was no other BB offer for room discounts, etc., just the ability to book Free Dining a year ahead.
The regular room discounts BB for resort guests didn't start again until 10/23/07, and that one did include some December dates plus dates into the next year. Maybe you'll have to make
another trip to WDW besides the one in September just to get a BB for December
Who knows what they will do this year though

, but you should be prepared in case there isn't a BB for you to book for December, but instead will be to be able to book FD for the next year (2009)- go, or no go
