I don't have a voucher in front of me with the terms and conditions but what it says is something along the lines that "value is lost in the event of changes or refares, guaranteed airfare rule excepted."

This was the case for a paper voucher I received in April 2008. It's possible things have changed since then. Consider asking the agent to read all of the terms and conditions or reading them yourself if you have them in front of you. I am absolutely positive that the language was as I described above as of at least April of this year.