Override properties in an Ant target

ant

Solution

<?xml version="1.0" encoding="UTF-8" ?>
<project default="all" basedir="."> 
    <taskdef resource="net/sf/antcontrib/antcontrib.properties" />
    <target name="all">
        <property name="prop" value="1" />
        <echo message="prop = ${prop}" />
        <var name="prop" unset="true"/>
        <property name="prop" value="2" />
        <echo message="prop = ${prop}" />
    </target>
</project>

Problem

We use lots of properties in our Ant scripts, run from Eclipse. I want to set up a parallel deployment which builds the project with slightly different property values, and deploys to a different location... deploy location is also a property. [How] can my new target update some properties to custom test values and then run the normal target to get the desired result? Simple sample script is very welcome, I only know enough Ant to get by :)

Original source