I've fed my snakes the same way and have had a lot of success with them making the switch. I start babies off on live hopper mice. I will increase the mouse size up to the point of a large mouse. I'll try to get them to take a rat pup (same size as a mouse) straight away. If the hesitate, I'll throw the rat pup in with some mice or mouse bedding to scent the pup. If that doesn't work, I'll wait til the next feeding and offer a smaller mouse. Then try to get them to take 2 smaller mice. next feeding I'll go to smaller mouse and little smaller rat pup as the second item.

Most of the time they switch right away but that's my process. Its worked for me and hopefully you can tailor it to work for you. Good luck