Dear Ram,
in standard there is no partner re-determination available. SAP provided this functionality in later crm releases ( I think with crm 7.0) which is now available for service requests, leads and opportunties. That means further that the partner re-determination starts only if you 1) have customizied this in PDP and 2) only when you change the mainpartner.
Otherwise you have to select the partner function "Contact" and push the button "propose alternatives". Then the new partner determination is triggered again and a popup should come.
See also:
http://help.sap.com/saphelp_crm70/helpdata/en/b5/d379615fe9493ca5693aac31e4a488/frameset.htm
and
Best regards
Rene