You can delete and reenter the Form 1099-R, allowing you to re-specify the parameters for the Simplified Method upon reentry. I haven't experimented with that, but I would guess that you would need to cut all of the dollar amounts in half, including the amount already paid, as if the pension was always half it's original size in all respects. But if the payer provided a taxable amount in box 2a, I probably wouldn't use the Simplified Method at all (although I assume from the title of the original question that the payer indicated that taxable amount is unknown.
