backward compatibility and function levels

Windows 2008 has bring two more function level to domain controller compatibility in Microsoft Domain World. More and more, I consider the function level or mode ( for exchange ) as part of backward compatibility that Windows has to deal with when new mode is co-exist with olde mode and replace the old mode eventually. The consequence of raise function level / mode is that you can not have a old version domain controller join in this domain/forest/exchange organization any more. That is it, raising the function level lose the backward compatibility when come to what version of domain control can join in the same active directory.

Of course, raising function level is not a reversible change. You can not roll back the change. The raise open up the advance feature the new version come with.

Note: Network clients can authenticate or access resources in the domain or forest without being affected by the Windows Server 2003 domain or forest functional levels. These levels only affect the way that domain controllers interact with each other.
Daniel Petri has an excellent article cover this topic.

http://www.petri.co.il/understanding_function_levels_in_windows_2003_ad.htm