Update
Who's your surgeon? This worries because your plan could change, and WLS could become excluded. I always advise people to put in the predeterminations as soon as possible. If you are denied, then you have to time to appeal. If you are approved, you can still lose the weight preop.
The lasted medical information I've read shows that there is no benefit of a diet between the time a need for surgery is determined, and the surgery itself.