"Career changer" sounds to me like someone who changes careers frequently, not as a one-time thing.
One of the
LEO discussions suggested "lateral entrant". It's pretty jargony, but it's the closest match I've seen among the proposals. Really, the most common thing to do in English would be to use a brief explanatory clause, e.g. "who took an unusual career path", "who came from outside the field", etc.
"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons