Hi Teja,
Migration is from older version of portal 7.0x to new versions 7.3x/7.4/7.5
In older versions the portal applications were not J2ee application but a different type and from 7.3 the applications are j2ee applications.
That's why when doing this upgrade you need to migrate your applications from PAR (Portal archive) the old files to WAR (Web Archive which is a standard) and it's the new file.
You have a tool in the upgrade process (also after) which is called PAR to WAR migration.
BR,
Saar