Greedo
Is your question a hypothetical one or based on experience?
While I don't spend any time in the Flight Levels (apart from an hour or so at FL140 last week), I do get to hear the jet-jocky traffic.
I would have said that when they get an ammended clearance as you describe it is usually only temporary due to traffic and ATC issues and they seem to be recleared to what I assume was their flightplanned level quite quickly.
For example, the initial clearance from "Clearance Delivery" might be to FL160 but on transferring to Approach/Departures they are recleared to FL360.
There must be some blow-pipe pilots in here who can speak with authority!
Dr 
PS: Interestingly, the guy on Clearance Delivery did not even hesitate when clearing the BE35 to FL140 last week.