Addendum:
Here
http://www.avianca.com/en-ar/faq.aspx#tabs-2 you find the following statement:
4. How can I change my travel date?
Date changes are subject to space availability. Depending on the Fare Type, you must pay a change penalty, which depend on the conditions of each Fare Type. If the original fare is not available you must also pay the different between the current and original fare. To make the date change you can contact our call center.
But it seems to be a secret how much the change penalty is...