You can feed rats anytime you decide to, I have snakes that have never seen a mouse in their life
Best way to transition would be to offer a rat similar in size to the mice you are currently offer, than once successfully switch (3 to 5 meals) you can upgrade size as needed.
I just picked up 2 bps, one of which was an ASF ONLY eater...I switched him to med rats with NO problems...Just offer similiar sized rats and you should be good...
Good Luck.
"I hate cold. I put on a sweater to eat ice cream sometimes." - Wolfy Hound
1.0 06 Pinstripe
1.0 08 Spider
0.1 05' 2300 gram Normal
0.1 06' 1600+ gram Normal
0.1 07' 1400gram Normal
1.0 10' Black Pastel
0.2 Cats
0.1 Dog