Welcome to the site! You could probably switch her over to rats any time, most say the sooner the better. Just find one about the size of the food she's currently eating. If she's on adult mice, then a juevie rat may be perfect, like stated above, go with the food to girth ratio. If she ate yesterday, give her about a week and then try feeding. She'll need time to digest what she's got in her currently. Feeding too often can be hard on the digestive system.
Some bp's take to rats no problem, you may need to rub a rat against a mouse to scent it, but I would just try the rat and see what happens. Enjoy your new bp, they're wonderful snakes! Feel free to post any and all questions you may have, we're all about helping each other here. Oh and post some pics! We love the pics!