You have to postpone for a specific date I believe, not just a voucher type scenario. I second @Kermit750 if you're not in a rush to apply it might be better to wait till you're pt-ing in your goal range till you signup .
@"Leah M B" I didn't signup for the July test but I was able to signup for the September test just now. Now I don't have to cry myself to sleep for missing the deadline!
their reply is going to be -- you can signup for September 2019 in March 2019 or whatever the signup date for the LSAT is. Just get request a refund for November if you still can.