The number of times a character is forced out of a career and then gets an opportunity to join later through the draft or an event is pretty limited. I suppose you could just let people who were expelled reapply, but I don't know that that is RAW either.
It's especially relevant in regards to Drifter, though.
Hmmm. The only part that actually talks about rank being reset within a career is when it's a change of assignment in Agent, Citizen, Entertainer and Merchant. Army, Marine, Navy, Noble, Rogue, Scholar and Scout explicitly retain rank. Drifter is not mentioned, but neither is any roll required to select one of of the Drifter assignments, so most of the text in that section (about what happens if you make of fail the enlistment roll) wouldn't apply. However, since any attempt to change assignment is automatically successful for Drifter, they'd start with rank 0 like Agent or Citizen. Which makes sense.
So... explicitly, a rank 2 Diplomat who changes to Dilettante (requiring a roll to do so) becomes a rank 2 Dilettante. But since all that really means is they get Carousing 1 if they don't already have it, it makes sense. Continuity of benefit tables also passes muster.
There really does not seem to be anything that says you lose your old rank when returning to a career, unless it's a new assignment outside of Army, Marine, Navy, Noble, Rogue, Scholar or Scout. Which are all brand new jobs, by definition. Retaining rank does simplify things.