overwriting ANT properties file - ant

Question

How can i overwrite some existing property with a newly created properties file?
Here is the required structure:
initially load Master.properties
generate new.properties
load new.properties and master.properties
run master.xml (ANT script)
The idea is that Master.properties generates some product version which should be replaced by new.properties. However, other properties in Master.properties should be kept the same.
Reading this does not help as i do not know how can i load the new.properties file
EDIT Here is ANT Script:
<project name="nightly_build" default="main" basedir="C:\Work\NightlyBuild">
<target name="init1">
<sequential>
<property file="C:/Work/NightlyBuild/master.properties"/>
<exec executable="C:/Work/Searchlatestversion.exe">
<arg line='"/SASE Lab Tools" "${Product_Tip}/RELEASE_"'/>
</exec>
<sleep seconds="10"/>
<property file="C:/Work/new.properties"/>
</sequential>
</target>
<target name="init" depends="init1">
<sequential>
<echo message="The product version is ${Product_Version}"/>
<exec executable="C:/Work/checksnapshot.exe">
<arg line='-NightlyBuild ${Product_Version}-AppsMerge' />
</exec>
<sleep seconds="10"/>
<property file="C:/Work/checksnapshot.properties"/>
<tstamp>
<format property="suffix" pattern="yyyy-MM-dd.HHmm"/>
</tstamp>
</sequential>
</target>
<target name="main" depends="init">
<echo message="loading properties files.." />
<echo message="Backing up folder" />
<move file="C:\NightlyBuild\NightlyBuild" tofile="C:\NightlyBuild\NightlyBuild.${suffix}" failonerror="false" />
<exec executable="C:/Work/sortfolder.exe">
<arg line="6" />
</exec>
<exec executable="C:/Work/NightlyBuild/antc.bat">
</exec>
</target>
</project>
in the above script, <exec executable="C:/Work/NightlyBuild/antc.bat"> will run Master.xml ANT script. This Master.xml will load up Master.properties:
<project name="Master ANT Build" default="main" >
<taskdef name="CFileEdit" classname="com.ANT_Tasks.CFileEdit"/>
<!-- ========================================================== -->
<!-- init: sets global properties -->
<!-- ========================================================== -->
<target name="init">
<property environment="env"/>
<!-- ========================================================== -->
<!-- Set the timestamp format -->
<!-- ========================================================== -->
<property file="Master.properties"/>
...
</project>

Answer 1

You should be able to resolve this by looking at the order in which you load (or otherwise specify) your property values. You probably don't need to override property values at all, which something not supported by core Ant.
Maybe you can split your Master.properties into two files - one loaded before you generate new.properties and one loaded after?
Maybe you don't need to generate new.properties at all.
Could you give some more detail on what you need to do?
Since you eventually fork a new Ant process (exec antc.bat), does that not start a fresh environment anyway? If it just loads Master.properties, those are the only properties it will have.
Not sure what your antc.bat does, but it's pretty unusual to exec Ant from Ant in this way. There are two standard tasks which might be useful - Ant and AntCall.
OK running on from your later comments...
Let's say that instead of doing this:
<exec executable="antc.bat">
you instead did something like this:
<ant file="Master.xml" inheritall="false">
<property name="Product_Version" value="${Product_Version}"/>
</ant>
I think that is getting towards what you want. You selectively pass specific values that you have obtained by loading new.properties. See the documentation for the Ant task.
If you still have the problem that you already defined Product_Version before loading new.properties, then I would say get the script you have that produces new.properties to output the version with a different name, e.g. New_Product_Version. Then invoke your master build something like this:
<ant file="Master.xml" inheritall="false">
<property name="Product_Version" value="${New_Product_Version}"/>
</ant>

Answer 2

May be this is a old question. Hopefully OP is reading this.
You can just use the ant task "propertyfile". reference
it can read properties from the file and write back updated values to them.