We also had a difficult time switching our bp from mice to rats. We tried rubbing a mouse on a rat pup, but she saw through that. So we tried the old "bait and switch" technique, which worked great. We thawed out a rat pup and a mouse, and first dangled the mouse in front of her to whet her appetite, and then when she seemed ready to strike, we switched the food to the rat pup at the last minute. Because she was already in feeding mode, instinct took over and she struck the rat and swallowed it down. After that, she has taken the rat every time it has been offered.
One word of caution is that once your bp is taking rats, you must always feed rats and not go back to mice, otherwise your bp will have to be primed to take rats all over again.