All of our domain controllers are Server 2008, but the domain functional level is 2003.
We would like to upgrade all the domain controllers 2008 R2 and also upgrade the domain functional level to 2008 R2.
We see no reason why we would ever add any new domain controllers in the future that are not 2008 R2, so that is not a reason to not upgrade.
However, if it causes problems with any of the company's line of business apps (some of which are very old from before 2003), we would might have to revert back to 2003 domain functional level.
If the issue was not noticed immediately, it would be a big mess to attempt to restore AD from backup.
Are there any kind of features/configuration/schema that any kind of application could be relying on the AD being at an older domain functional level?
Is there any way to test for this in advance of changing the domain functional level?