I actually need a General Testing Strategy or a Methodology document that needs to be followed by a testing team whenever an external application (used by the organistaion) or when a database system is upgraded.
What's the difference with another test project?
I guess you also investigate the risks associated with the upgrade, and based on that determine what needs to be tested in depth, what needs less testing, what doesn't need testing...
As an input I would use the test strategy that existed for the system, as this will indicate the important aspects of the application. Regression testing the existing seems a good start to me.