ant-contrib’s Variable task can do this:
<property name="x" value="6"/>
<echo>${x}</echo> <!-- will print 6 -->
<var name="x" unset="true"/>
<property name="x" value="12"/>
<echo>${x}</echo> <!-- will print 12 -->
Not recommended, though, it can lead to weird side-effects if parts of your Ant scripts assume immutable property values, and other parts break this assumption.