I've been able to successfully convert all of mine over to rats using the "tough love" method. If they have good weight on them, they only get offered an appropriately sized rat once a week, and are never offered a mouse.
A happy, hungry ball python will eat what's offered to them. They just have to get hungry enough. I've not had any take more than four weeks to finally take a rat. They may refuse again the following week, but they will usually eat again the week after that and from then on out.