Because your birds have been outside, they slowly became used to the dropping temperatures and a sudden change to warmer temperatures may do more harm than good. You can move them to a new location, but try to make sure that the temperature is fairly close to what they are used to and then you can gradually increase it until you get it up to where it will be more comfortable for them. You mentioned that you want to move them into a garage, so you shouldn't have a problem keeping a door or window open for a few days - then close it - then start to add heat. Good luck.