I have to say, and it may be naive, but how can they change?

It's a sexual preference - you cannot change homosexuals to be straight just as I believe you cannot change paedophiles not to like children (and I am NOT saying homosexuality should be changed or is a bad thing, that's a persons right to choose).

Whilst you can teach them how wrong paedophilia is in prison and mental units you cannot take away the "urge". Short of castration (and I do not see why they should not be castrated after taking a childs sexuality) nothing can be done to ensure they do not lust or re-offend.