There are a few things that you need to be aware of when it comes to getting credit from non-refundable tickets. Among them:

• You pay the change fee as an additional payment; it is not deducted from the value of the ticket.
• The change fee is paid when you book the new flight, but you have to call to cancel the old one prior to the old departure.
• You must book a new ticket under the same name as the original ticket. This new ticket can be for any price.
• Should the new ticket value be lower than the original ticket value the residual credit will be issued as a TCV.
• The residual TCV value can be used to book as many tickets as you want for any passengers (not just the original name).

Assuming you've paid for a $1000 ticket that comes with a $150 change fee, you would pay an additional $150 to rebook the initial ticket. After accounting for the cost of a new ticket in your name(e.g. $200) a TCV would be issued for the remaining credit ($800, in this case). That remaining credit can be used over the one year life of the TCV for as many purchases as you can draw it out over, and those purchases can be for tickets in any name.

Up until late 2007 TCVs were one-shot deals. The current policy took effect on November 12, 2007.

If you have a non-refundable ticket in someone else's name that you want to use, the most cost-effective way to do so is to find the cheapest intra-Texas flight you can. Book one of those as a one-way, paying the change fee and using ~$50-80 of the credit value. That will leave you the balance to use for booking in your name (or a 3rd party's). Check for city pairs where CO competes with WN for the cheapest options. IAH-AUS/SAT are often pretty low.