As far as swapping one head bolt, it shoul be ok. I agonized about doing that on my GTO because I needed the stud for the starter cable tube. Folks assured me it would be ok and there was no problem. The head is stuck on there pretty good. I wouldn’t even worry about draining the coolant. Just pull the bolt, figure out what size you need, install the correct one and torque it. I would just torque that bolt and call it a day.