Emirates/Skywards are great a recrediting miles back to your account if you decide not to go ahead and book. I remember a while back I wanted to upgrade, but did not have the miles in my account at the time, they held a booking for me even. Skywards have to be the most flexible airline when it comes to spending miles. Book it now if you have the miles, and cancel nearer the time. I have just cancelled an upgrade for Friday, I rang them, and now I am back in J class and checked my account less than 30 minutes after and the miles are back into my account and the ticket has already been re--issued in J. Top marks to Skywards for this!