They are currently offering a credit for purchasing SSR. It is a $10 credit but it is not called MB-because you don't have to sell the first year's points back.
If you are seriously considering buying I would call and find out what is available. There may be points available at the other resorts that offer MB but there is no way to know unless you check. Availability of those pts if going fast.
Are you paying a huge amount for your FTP? Are you staying at a
DVC property?
I suppose there could be chance that you could buy into DVC and still find availability for September but you would have to cancel your FTP and apply that money to DVC. I'm sure DVC would be able to tell you exactly what you should do.
My guide, Jim McCoy, is working today until 9pm. His number is 1-800-827-7140, if you want to give him a call.
He should be able to advise you on whether you should purchase now and change your FTP or just wait until you are at WDW.
Good luck with whatever you decide to do and let us know how it all turns out.
Holly