I had a sort of similar situation arise in 2009. BF and I had started planning a 3-week trip to the South of France in March; even though the trip wouldn't occur until early October, we had flights and housing booked and non-refundable deposits paid by late April.
In July, nephew called to say that he and fiancee were going to have a destination wedding in November, because they had been able to get package booking at a great discount then (original date was spring of 2010). I had to give my regrets to nephew because I had used up a) all my vacation time and b) all my vacation budget for the year on our upcoming fall trip.
Nephew, his fiancee, KidSis (MOG), niece and our Dad were all fine with that - and I have a beautiful DVD of the wedding, from rehearsal dinner to the reception's end.
My niece promised me she would get married in Alberta or BC - and she did, in Cremona AB on July 6.