I'd get them on f/t first then switch them over to rats. Live rats are pretty dangerous, so I just don't even want them in the same space as my ball pythons.

When switching from anything to anything I usually start by skipping a feeding. Hungry snakes are happier to make a switch. If they still don't like it, I give them their usual meal the 2nd week, but then I skip another week and try the new food again after that. And so on. Older snakes generally take longer than younger snakes.