yeah it happens all the time, probably less so in medicine because of the lengthy training period but even in medicine too. here are some stories i know...
a business man who went back to medical school to become a psychiatrist in his early 40's
lot's of img's who come here and have to repeat residency just because they wanted to move to usa, one was even so far as completing rhematology residency in his home country.
i also had a college buddy fellow pre-med who worked his but off, got into med-school, then decided it wasn't for him quit the medical path altogether and followed his passion to become a wrestling coach. he was a top wrestler in his class as well during college.
a heme-onc doc who went back to fellowship and became an ID doc.
with you though medicineman it sounds like you want to do pain... there is another kind of inspiration too... sometimes you just have to wait on the train for awhile until you can reach your destination... and anesthesia residency is a fine train to be on to go into that. think about that to if you really need to switch since the destination might work out in the end. i can think of that scence from the movie kung-fu where he is made to sweep all the temple steps and light 1000's of candles in the temple, and the master asks "how long have you been here" he replies "not very long." the master says. "now you are learning." then he finally gets to start learning some kung-fu
lol