Some more info for people who have an existing reservation they will need to change once FD drops. So, if you click on "Change Reservation", you get this page:
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If you click on the "Change" under Vacation offer (red box), this may show up:
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Which, started to freak me out!
But if you actually click on the "change" under room type (pink box), it brings you to the page to add an offer:
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The FD offer will show up in the orange box if/when it becomes available.
For my reservation, it would allow me to change room type or resort, if available. But it would not allow me to change dates (not that I wanted to). It could be because I already have DME flight info in the reservation, or just the way the package is set-up. So if you need to change your dates, be prepared to possibly need to either book a whole new reservation or call in.