There have been a few threads on this. I would do a search on Vouchers or Travel Certificates and see what you find.

As I understand the rule, your ticket is put into a voucher which you can use to purchase a future CO flight (at any fare class for any pair of cities). The only catch is that the ticket must be in your name.

As for the change fee it will depend on your particular ticket I would guess $200.00 or so.

The only thing that could be a wrench is that in your case this is a part of a ticket you already used half of. So that might change the rule and maybe your TA was right. I would double check and call CO