I am a seamstress and costume maker-and they can definitely alter the dress down 2 sizes. The only thing is...the alterations if done in the bridal shop will cost a lot.
If you are going strong and are sure that you will be a size smaller or two-order ONE size smaller-but not two just in case.
Did you say NEXT September...as in 1 year and 7 months from now???? Or in September 7 months from now? If it is the year and 7 months...I would insist that she wait a while before having you all buy the dresses-too many variables can happen in a year and a half's time-pregnancy, weight losses and gains, a new crop of dresses that she likes better...
I feel for you...I am in my SIL's wedding as matron of honor next June...and I have a strange feeling that I am going to be a lavendar nightmare.
I have been putting a bug in her ear to let me and the bridesmaids all wear the same color and fabric-but different dress styles-my taste is totally different than the other two bridesmaids.