Using AU in a campaign is a relatively 'seamless' process.
New Races - Nice but not necessary to use to play an AU game. Add some or all of the races to a existing campaign is no biggie, beside explaining why this new race suddenly appears in the campaign.
Classes - the classes are slightly more powerful than PHB classes but not so much that you can't have PHB and AU classes in the same campaign. You might decide that Wizards and Magisters (for example) are too similar in concept and may decide to use one or the other.
Feats - feats derived from 3.0 have their own AU spin to them so you will have to decide what version of the feat to use. You can use AU and PHB feats together with little problem.
Skills - some skills are done a little different than 3.0/3.5 in that they are opposed checks instead of target number DCs. Pick which version you like to use.
Magic system - the real strength of AU. The magic system is very flexible with simple/complex/exotic spell lists, heightened/diminished spells, weaving spells and spell templates....yet not so different that players will feel that they are learning a completely new spell system. The only hiccup may be if your campaign uses divine magic....the AU magic system doesn't differentiate between arcane and divine spells. So, you either use the divine magic from the PHB and explain the difference between divine (PHB) and arcane spells (AU) as the way that divine and arcane magic work. Or, you make conversions of divine spells to the AU system.
Other interesting aspects ...the adoption which isn't hard to do. Hero points and the AU dead/dying/unconcious rules are easy to use. Alignment will have to be a decision. AU doesn't use alignment...there is still good and evil in the world and law and chaos; but these no mechanical impact the the game. So, spells, special abilities and magic items dependent on alignment are gone. Or you can still use alignment if you wish and keep these things if you want.
Hope this helps. Check out
www.diamondthrone.com or Monte Cook's forums to see how AU/AE fans are using AU/AE in their games.