Question

We’re currently running a server on Compatibility mode 8 and I want to update it.

• What are the implications of just going in and changing it?
• What is likely to break?
• Is there anything that checks the data will survive before I perform it?
• Can I rollback to mode 8 without performing a restore and without loss of data?

Answer 1

If you’re going from 80 to 90, the differences are minimal. Going from 65 to 70+ can cause severe impact (NULLs are stored differently).

Implications – your SPs can return different results than you’d expect Likely to break: functions, SPs Data should survive; nothing in there should affect things.
Moving from 80 to 90 and back only takes a few seconds. Yes, you can move back and forth.

http://msdn.microsoft.com/en-us/library/bb510680.aspx

some gotchas: http://mapamdug.blogspot.com/2006/03/sql-server-2005-gotcha-1.html