You can just change your basal rates in Loop to half or your normal basal settings (write down the old ones first, or take a screenshot of them before changing). And on surgery day, turn off Closed Loop and just run it in Open mode with the changed rates.
…and how do you know what your correct rates are when Loop is in Open instead of Closed?
Because you are an FUD member and you are paying attention to threads like this:
And because you are doing that well before the surgery - not the day of - you know what the correct basal rates are when Loop is Open instead of Closed.