normally it should work to smoothly upgrade from 4.8.2 to the 4.8.11 RC kit without your CF/USB beeing erased - BUT backing up your images is probably not a bad idea - because it is still 'only' a RC kit and people have tested this not too heavily in all combination and on all boxes. This is what an RC kit is about - to find out if it is ready to be released.
Ciao
gutemine