I'm looking for a word that doesn't have such negative connotations as defector, dissident etc, but still implies actively turning from one set of ideas in favour of another.
Convert might work (as a noun); it can be used to denote somebody who has converted:
1 a : to bring over from one belief, view, or party to another
// They tried to convert us to their way of thinking.
1 : to undergo conversion
// He converted to Islam.