If I buy an A tag for an elk zone but don't use it, can I hunt the B tag hunt instead?

Only if you're a youth hunter. The 2011 Big Game Seasons and Rules explains: Resident youth hunters ages 12 through 17 who purchase a general season elk zone tag, may participate in any A or B tag elk season within the specified zone, regardless of whether they purchased an A tag or B tag. This opportunity for resident youth hunters does not apply to controlled hunts. All other season, weapon restrictions and commission rules apply.