If you're using ODBC, the transition should be relatively gentle, just change the settings in the ODBC.
If you used a direct connection (or another means), and the name of the server changes, you'll need to change the connectivity for every report. Try to convincec them to use the same name if you aren't using ODBC.
I haven't had to do this in large scale recently, I believe there are utilities to handle changing the data source now, hopefully someone will chime in with that part.
If you have structural changes, you'll have to either alter the reports or the underlying data sources (SP's/Views).
If you used Views/SP's, then you are fortunate, the Views/SP's get changed and the reports function.
I always suggest using Views or SP's for reports.
-k
kai@informeddatadecisions.com