I have an existing Windows DB2 database that I'd like to move onto PostgreSQL. The users for the application can live with a weekend's worth of downtime. What's the best way to do that?
This is how I would do it:
Create a test system with a DB2 and a Postgres server. Try the migration (ideally with some real data from one of your backups). Make sure it works. Create a script that automates the process - and make sure that works, too.
Then, on migration day, set up a redirection to a static HTML page, stop the server, make a backup, run your script. Check that everything went ok.
Make sure, that it works, too.