Last time we tried to extend the dates of a voucher in that way, the computer said no as the expiry date had been fixed. They used to have that flex, but I think they closed that off when they made the changes to allow UC use for Red member. Now if you try to extend them beyond the standard booking window (circa 11 months) from when they were due to expire, then that is the hard cut off.

Keep in mind that with it only being 50% off in UC for a Red member booking (which would also cause a reprice if you try to change dates within voucher expiry dates, but with a change of status to Red), using the voucher as an upgrade voucher is often a better use for Upper Class redemptions. One voucher would price an UC return redemption for PE points, which often is better than getting 50% off.