Well, I am sorry but I have never heard of a Pre diet before the surgery, but then again I got screwed by my insurance anyway.
I hope you get approved. The only advice I have is make sure you have an approval letter before surgery. My doctor sent in all the necessary paper work for mine, told me I was ok, and good to go for the surgery then 2 weeks after I had the Lapband, I was at home and got a denial letter from my insurance. NICE HUH??? Yeah, well Its been 5 years and I am not about to pay 70,000 for that doctors mistake.
(I had complications in surgery so I was in the hospital for 11 days, long story)
Anyway, I am not trying to scare you, I am just saying, Please make sure you have an approval letter from the insurance before you get the surgery!!
GOOD LUCK and I hope all goes well for you...