How hard the switch is almost always depends on the snake. Personally, I've never had an issue with it, but I always make the switch when the snake is young and is programed to eat anything in order to put on size and weight.

Some people want to feed mice until a snake is into adulthood then get tired of having to give multiple mice per feeding. When they try to switch them over as adults, it can be a much bigger probelm. Some BPs can be pickey and the best tip I've heard is to get some mouse bedding from a pet store and rub it on the rat before presenting it. Before you mess around with that, though, you should first try a straight switch.

As for the size question, that's kind of hard to answer because everyone seems to have a different size scale. Off the top of my cranium I would say that any snake that can eat a large mouse can probably handle a small rat in the 50 gram range. This is another reason a digital scale is an essential piece of gear for any snake keeper to have. Weighing the prey before feeding takes out all the guess work.