If you love it, and work really hard at it, it will really happen, I believe. I'm living proof.
The growth of technology is such that it is not possible today for a nuclear physicist to switch into medical physics without training. The field is now much more technical. More training is needed to do the job.
It disturbed me that the music industry had gone down the drain, even though people were listening to more music than ever and from a greater diversity of artists.