Depends on what type of mouse they're eating. If the snake is taking FT mice, try offering a PK rat of equivalent size. I wouldn't go all the way back to live, since you'd just have to get them back up to FT later.

All my snakes take either pre-killed or frozen/thawed prey. I have 5 ratters out of 15, and I'm pretty confident that my young corns will readily take rat pups once they're big enough for them (so that's three more on rats).
I've converted three snakes so far that were mousers to rats. Two took the rat the first try and haven't looked back since, the third took a bit of convincing to take the rat but now is going strong. I've tried a few times with two others, but to no avail as of yet. The key to any conversion of prey is patience and consistency. Don't just give in and offer a mouse because they don't take the rat - be strong and wait the week, then offer another rat. Tough love seems to work best.