EDIT: I clearly was not thinking when I wrote this post. My J-pouch procedure was done in two steps and for some reason I assumed that yours was as well. Hence why I repeatedly mention the functional changes that come with the J-pouch. I am guessing that this is step two out of three, and so most of the information that I have provided will not apply. Regardless, my overall point still stands and I, personally, would not alter my diet around a surgery and you should discuss the pros and cons with your doctor.
I want to preface this by saying that I don't intend to frighten you with anything that I say, as I know that an upcoming surgery causes plenty of stress, I just want to inform you of some of the changes I experienced after my surgery. If you take one thing away from my response, make it this: please discuss this and any other concerns, dietary or otherwise, with you surgeon and gastroenterologist. They will likely give you a pre-written diet plan for about eight weeks post-op that will consist of low fibre and high protein foods in order to reduce the load on your recovering digestive system.
Firstly, I would not recommend attempting to transition to a new diet around the time of your surgery. It may not harm you in any way, but I would avoid taking any risks. You should definitely discuss this with your doctor. I'm not knowledgeable about many of the diets out there, but regardless, I would not alter my diet in any significant way in the lead up to surgery. Post-op, I reintroduced foods gradually as certain foods may irritate your gut or cause discomfort. I made sure to do this with only one food at a time so that I could keep track of whether or not a specific food caused issues.
Secondly, do not underestimate the toll that surgery takes on the body. It is very physically taxing and may take longer than expected to recover from. Additionally, your gut will have been cut, stitched, and moved about by the surgeon. It takes time to recover from that. The J-pouch will likely feel very different to your current digestive system, and especially for the first few months (beyond six months in my case, and it still occurs on occasion) you will feel pretty much everything that you eat, especially if it is something that you either have not tried since surgery or if it is a difficult to digest food such as uncooked vegetables.
In regards to the pros and cons of altering your diet, I personally do not see any pros in altering your diet if your surgery is relatively close. You will most likely be recommended a low fibre, high protein diet for roughly eight weeks post-op, the same diet recommended after the ileostomy was formed (this was the case for me, I don't know what you were recommended). If this does occur, it's likely that any adjustments that your body makes to accommodate your new diet will be lost/greatly reduced by the time you have come off the low fibre, high protein diet, not to mention adding foods back into your diet individually.
While that was probably quite a bit of information, I feel that it is important to consider the likelihood that you will gain any great benefits from changing your diet in the lead up to this surgery unless your surgeon or gastroenterologist has recommended it. I personally do not see any pros in doing so, and my experience was that the J-pouch was quite responsive to what I ate in the early days; that is, I would notice changes in my digestion (loud churning and discomfort particularly) when I ate something that the J-pouch perhaps was not ready for. This was usually things that can cause gas, such as high amounts of vegetables.
Final disclaimer before I wrap this up: I am not a doctor and please do not take what I have said to be recommendations or medical advice. You need to discuss this with your surgeon and gastroenterologist.
I wish you all the best with your surgery and hope you have a quick recovery. Please let me know if you have any questions or if I was not clear.