Hi, Lillyjgc!
You are absolutely right about the Moon applying to the natal Sun. Thank you for pointing it out. Mercury messed with my thoughts, it seems, but I have already corrected the mistake. The reasoning behind the suggestion about the Moon being in fixed signs comes from the idea that this would help to keep the surgeon's hands steady. Also, this is a surgery where the outcome is known, which is not at odds with a fixed Moon, whereas a mutable Moon would be convenient if the surgery was one of exploratory type. A cardinal Moon would be best, I think, for those cases when the patient might have complications due to anaesthesia and it's necessary for the surgery to be done fast.
You are right about surgery changing something, but right the moment a knife cuts your skin and gets inside of a patient's body, that change has already started, right? And hopefully, it will be from negative to positive, when someone's health is restored by someone else's ability, and the help of the wonderful energies from above.
Cheerio!
Carole