if the snake ate rats before, then you offer mice. and now will only take mice over rats. means the snake likes them better than rats ( no brainer). if this is a snake that you plan on breeding. feed it what it likes and does well on (mice). if it's just a pet and getting to breeding size isn't a concern, wait the snake out. the snake prefers mice over rats and not going to eat rats until it's really hungry. a lot of my big females made the switch themselves. meaning once they got larger (over 3000 grams) they lost interest in mice after feeding on them their entire life, and now won't take them any more. now they are slamming rats that they wasn't interested in before.