PostgreSQL: 8.3/8.4 interface chages?
Unfortunately, without knowing your system (inside and out), it's going to be difficult for anyone to tell you how an upgrade is going to impact you.

I'd recommend checking the release notes to verify that the upgrade path won't mess up the application.

Here are a couple changes that I noticed that you might need to know about:


Remove ipcclean utility command
Changes in the TRUNCATE and LOCK statements
SELECT DISTINCT and UNION/INTERSECT/EXCEPT no longer always produce sorted output 
Disallow LOCK TABLE outside a transaction block
Remove support for the (insecure) crypt authentication method
Modify the ALTER TABLE syntax to allow all reasonable combinations for tables, indexes, sequences, and views 


I'd recommend trying this on a test machine first (if possible).