A lot of times the rules for a fare class change but the text that's printed out with that fare class isn't updated. There are several deep-discount AA fares whose fare rules said "non-upgradeable" for a couple of years after that restriction was lifted - and for all I know still do, I haven't checked for some time.

The most reliable source is usually the section of thye airline's Web site that discusses upgrading, as they tend to update that first when the upgrade rules change. That said, as posted by flysurfer, not every agent knows all the rules, etc. You may get away with requesting an upgrade you shouldn't be able to get, or have to fight for one that you should be able to.