Lots of players with different skills play in several positions throughout their careers. I wouldn't say though that the qualities of full-backs who specialise in that position - on the whole - makes them especially interchangeable with the CM position. It depends entirely on their individual skill-set; some FBs are better suited to doing a job in CM, some aren't - (you wouldn't have Evra, Cole, Bosingwa, Clichy in there unless you could help it, for example). By the same token, you'll find CBs that move into midfield, CMs who move into defence, strikers who move into midfield, strikers who move out to the wing etc...
Aurelio's one I think could do a job in there on a temporary basis mainly because competition isn't exactly fierce in there at the moment.