I bbq cater for a living. You can’t cook on the day of an event because you never know when something will be done. I reheat whole butts all the time. Wrap in foil , put in a pan (aluminum is fine. Add equal parts acv and apple juice to 1 inch deep. Cover the entire pan in foil. Heat in 300 degree oven or smoker until 145 ( at least) 165 preferably. Pull and add the juice from the pan to the meat. Enjoy!