Resolution:
You need to update the bin\Properties.cfg file.
For example, if you want to start IM Server on 7501 and hawk is currently running on 7475, you would update the file as follows:

Bus.Default.Class: com.tibco.share.bus.BusTibrvdTransport Bus.Default.Port:    7501
Bus.Default.Service:    7501
Bus.Default.Daemon:    tcp:750        

Bus.Hawk.Class: com.tibco.share.bus.BusTibrvdTransport
Bus.Hawk.Port:        7475
Bus.Hawk.Service:    7475        
Bus.Hawk.Daemon:    tcp:7475

Just by changing the above mentioned parameters in the Properties.cfg, only IM server starts on the new port. IM Engine still starts on the default(7500) port.

For starting IM Engine on the port 7501, set the Service field in the Channel to "7501". Also in Job Creator create a shared parameter wherein you specify the host as "localhost" and the port as "7501"