It depends on the individual horse and her temperament. If she is easily stressed in general, this may stress her quite a bit. If she is pretty easygoing, it may not cause any problems. You don't say how she is housed at this point, but try to get her back to a similar arrangement. If she is out with other horses, try to turn her out with one other (easygoing) horse in the new group for a week or so, so that they can bond and she can adjust. Then introduce her to the whole herd. Otherwise, try to stay calm yourself and recognize that there will be an adjustment period.
