ANT update version number in file but not in script

Question

I'm currently working on a script that automatically increments version number. The version is in major.minor.build format so I can't use buildnumber. I've managed to increment the property in file like this:

<propertyfile file="./build.properties">
  <entry key="ver.minor" type="int" operation="+" value="1" pattern="0"/>
</propertyfile>

This works but ver.minor is not incremented in script. <property file="build.properties"/> also dosn't help.

After some googling and forum searching I hope that I'm just stupid and that I won't have to use two ant files or an external tool.