Update system property in ANT build step

Hi, I have created a system property in Step 7 Build Parameters and given it a default value. I would like to assign it a different value during an ANT build step in Step 3. I can reference the system property value, but can't seem to change/update it. What is the correct syntax?

ANT build file snippet (this does not work, although ${svn.last_changed_rev} prints the correct value):

<property name="system.last_commit_rev" value="${svn.last_changed_rev}" />
<echo>last_commit_rev: %system.last_commit_rev%</echo>