Moving from SJ Family Suite into a higher category while possible might not happen given several category upgrades normally occur more often in the lower cabin categories such as interior or ocean view and less often in suites since they are much fewer in number and are usually booked well in advance. IMHO save the price difference and get a cabin assigment in a good location amidship. Some higher categories such as SF while offering alrger aft balconies are actually smaller overall than the SJ.

You could of course pay the extra amount and take the cabin guarantee option and still wind up with the SJ category so upgrades aren't given on every booking all the time, especially if by "December" you mean over the Xmas & New Year's holiday season when occupancy on the Star is likely to be high.

Happy sails!