The question still remains, what "they" means:
Policies regarding Transportation Credit Vouchers issued for 1) Denied Boarding, 2) Service Recovery and 3) Residual Credit are being changed for TCV's issued on or after December 15, 2011. Before Dec 15th..rules remain the same. TCV's can be transferred with knowledge of the voucher number. Also, up to 3 vouchers can be combined to the purchase of a ticket. After Dec 15th..customers receiving a TCV will not be able to transfer it to anyone unless that person is traveling with the recipient in the same PNR. And only one TCV can be applied to the purchase of a ticket.
(Am only providing this information to clear up possible misinformation from the DM line
agent. I am not a spokesperson for Delta.)