Portal Upgrade from 7.0 to 7.3 need to customize par to war

Dear Team,

    we are planning to upgrade the portal 7.0 to 7.3

       Now SRM standard content only available in Portal , and par files customization

         From portal side what are prerequisites need to take for portal upgrade  7.3

                  Once the upgrade is happend all the standard content will get override , but thinking par files

how to convert Par files in to war file in step by step process

          I know "par migration tool" is available in portal

                  but how to do using that ,  need to place par file on desktop , before upgration need to take backup of par files ?

            Any thing else to take up back up from portal side ?

Thanks,

Renu.