Thank you all,
We have successfully upgraded our portal to 7.3.
The solution worked here is to install additional application server using latest SWPM - SW Prov. Mgr. 1.0 SP04 for systems based on NW7.1x.
During the installation of additional application server we kept same instance no on all 4 DI's and the intelligent SWPM did rest of the work.
Thanks again for all your support here.
Regards,
Rbk