I own only B shares. The better question is why anyone would by an A share when they cost more and are inferior in every way?
If you own A shares, I recommend buying B shares to replace at lower cost and then waiting a month and then selling the A shares. If your shares are above water like mine, you don't need to wait the month (for tax purposes to claim a loss).
You might not need the month since they are different investments, but I wouldn't take a chance with the IRS... If in doubt, ask an accountant.
Ditto on the own not trade, choosing the B shares because they are exactly the same except superior voting rights. They should trade for a slight premium as the voting rights make them superior to the A shares. I believe managment is supposed to be working on converting the B shares to A shares this year. That will erase the difference.