You do know that you actually don't ever need to switch your snakes to rats right? A ball python can live it's entire life feeding on adult mice. I have several males and females that are adults, great breeders and only eat mice.

I'd say if you want to switch to rats for your own reasons (convenience, cost, etc) then wait until you think your snake can comfortably consume the size rats that you have access to.

Hope this helps.

-adam