Another possibility if its not super critical is single point turning sometimes. You setup just like you were going to cut the thread, and adjust the threading tool to the existing thread by moving the compound. It might not be practical if all you can grip is the bolt head, but if there is enough length so you can center drill the end first to support it with a center even that is sometimes possible. I look at all of these type solutions as non-critical sloppy answers. The right answer is to buy a or make a new threaded item if its very important.
One I saw done where a fellow wanted to preserve as much thread as possible he singled pointed it using a hand ground threading tool turned upside down. No, not like Joe Pi. He used it as a forming tool and straightened out most of the thread with a big heavy rigid lathe. Then he flipped the tool and skimmed just the lightest amount off only the area of the damaged thread. Probably not practical on a grade 8 bolt, but then you shouldn't be doing anything but replacement on an application that requires a grade 8 bolt. If I recall the application in this case was a grade 2 field bolt used as a shear pin in a piece of PTO driven equipment. Not sure why it was important to preserve the thread that way, or if he was just an OCD farm mechanic. Or maybe he was just showing me a trick he knew how to do.
Farm mechanics are an interesting lot. Another one taught me to weld muffler skins with a coat hanger and a cutting torch.