Well, yes and no.
Part of the reason for a "new case" is to prevent people from ruining their rifles. Theoretically, you could use a new powder to soup up the .308 beyond the 30-06. Prbably be fine in most bolt guns, but any of those loadings would tear the hell out of a M1-A and many other semi-autos out there … not just the heavy bullet loads.
Keep smiling ... it'll just make 'em wonder what you're up to...