Need Booking Advice

[AmazingBagel]

We're planning a WDW trip for next fall and will probably book it for early December. However if we could get free dining we would switch it over to Sept or Oct (since free dining probably won't be offered in Dec). If there is no free dining we'd definitely prefer December vs September. How easy is it to switch dates once it's booked? The other option is that we would book it for Sept. and hope free dining comes along. The problem with doing it that way is that if no Free dining is offered we may be stuck with the September dates or left with little choice of December dates.