Congratulations on your future surgeries! I was on a pre o p DIET for the last few months required by my insurance and doctor and I have lost 40 lbs. as of yesterday. My surgery is next Friday and I am now on the pre op liquid diet, I began 4 days ago and I am not going to lie: it sucks! Each surgeon is different in what they ask you to do for pre op, but mine wants 10 days before surgery on liquids (which consist of ISOPURE zero carb drink...30 oz a day.) I have lost 10 pounds alone, since Wednesday and I know why, LOL! But I am just looking to the future after being sleeved and know how hard I have worked up to this point, following ALL the instructions, and plan to continue doing what I am supposed to do.

My pre op was 6 days of 2 shakes and a meal of meat and veggies. Then 3 days of liquids being any liquid but at least 3 shakes. I personally like the EAS Advant Edge chocolate shakes the best but also like premier protein but it makes me queasy some times. Personally I prefer the pre made. Also, some surgeons don't require anymore op. It varies greatly from one to the other.

Congrats on being approved!
The number of weeks for the pre-op diet usually varies with your BMI. The higher the BMI the longer the diet.
My surgeon wanted me to do a 3 or 4 weeks pre-op but since I lost weight on my own I only had to do two weeks. My BMI was really high, for my first consult I was around 53.
But all surgeons are different and will have different guidleines.
My advice would be not to stock up on protein powder because your taste might change post-op. It did for me and I had many containers full of expensive powder I gave away at the hospital because I couldn't use it anymore.
The pre-op diet is not that hard, the worth is the three first days but after that it will be easier.
Good luck in your journey! take care