If I delevel my 40 toon to 39, can I redo the 39 quest and pick a different occupation than the one I chose, that I end up not really liking so much???
yes. ONLY if you have not completed your class change (that is, if youre still the class you were at 39). if you have successfully changed classes, no.
Well ... there still are people that do not know some "basic" stuffs that are obvious for others, even if they played few years and are experienced. Suddenly you got some question, you are not sure about anything, you do not know answer to, and you ask, I do not see anything wrong with that.