Have you ever upgraded onboard?
 

I'm sailing on the Solstice on Sunday, and I've noticed that the booking is very light. I'm not sure if it's because it's just after spring break, if it's the reports of Norovirus onboard, or because the Allure sails from the same port on the same day.
It looks like all of the stern facing balconies on deck 7 are vacant. Is is ever possible to be upgraded while onboard(for a price of course) or does it impact their staffing etc and make it too complicated?

If the cabins are available, they'll be more than happy to upgrade you, for a price. I've never done it before because every cruise I've been on has had a sign posted at the purser's desk that they were sailing full. I have heard of people being able to do it, though.