I had to do that a year or so ago. Somehow a large number of my images got labeled as RM instead of RF. No idea how that happened, but it doesn't matter now. Contact contributor services and explain what you want to do. As jatrax said, send all the image ID numbers along with your request. They will change them over for you. Only exception would be if any have sold as RM or if any are on hold by an Alamy customer with a pending RM purchase. They'll let you know if any fall into this category.