I agree. Scenting nightcrawler pieces is probably the best way to go. They might not take to it right away but if you're willing to put in a little effort and can accept the fact that some of your snakes might not eat for a couple weeks during the conversion then it should work fine. You can also try offering a slug and the right after it's done swallowing offer it a scented worm piece. Once the feeding response is initiated snakes are more willing to accept food that they wouldn't have eaten otherwise. After doing that a few times you should start having luck without using the slug at all.