We setup Exchange 2007 in an environment that we later decided we wanted to have Exchange 2003 in as well. Our reason was to utilize the old IMAP migration tools, but for the purpose of this post thats not relevant. I have removed Exchange 2007 "cleanly" and also removed the "services\exchange" branch in the configuration area within adsiedit. I have also removed the two domain Exchange system objects areas under the domain tree. My question is about the schema. Is it supported to install Exchange 2003 into an environment that once had 2007 native but now removed? Of course, I'm aware it is unsupported to add 2003 to a native 2007 org, but did I remove enough to call it a "clean install" now? Please dont say to demolish AD and rebuild the forest unless you have documentation or logic to support it. I actually tested this, and it worked fine (I watched the install put everything back), but I noticed http://technet.microsoft.com/en-us/library/aa998313(EXCHG.80).aspxmentions that schema extensions aren't undone during the 2007 uninstall. Will I have problems in the future? Does letting these "extensions" float around during the 2003 install harm anything? 2003 didn't seem to notice, and then re-introducing 2007 went fine as well. Anybody have any article or solid thoughts on this? I need to sell my case that this is ok and will lead to a stable AD environment; or if it doesn't, I need to know that too. Oh yeah, by the way, Im focused on the schema here, but if there are other factors to consider please let me know

A colleague replied with the following: -----Original Message----- Subject: RE: 2003 after 2007 - schema or other affects... So I didnt see you already tried the 2003 installer, well then you should be in good shape...happy hunting.... -----Original Message----- Subject: RE: 2003 after 2007 - schema or other affects... Since I Am not a blogger per say I will keep this here When you install Exchange 2007 into an 2003 AD you extend the schema hence you are adding to the core structure of the schema structure. Adding is the imperative word..you are not overwriting or manipulating any values already present. My theory (founded in some RFC I cannot remember the number of) is the change control mechanisms in an LDAP compliant schema where you are not allowed to make modification that causes backward compatibility issues. This means that when you add/extend the schema for 2007 you will also put in place what's needed for Exchange 2003 to live in that environment. Now there is one more umm (maybe two) obstacle to this beauty: MS have(had) no publications saying how they validated schema numbers/version when the installer runs, but they started out just looking for "greater than #" in their installers. (the other possible issue might be if MS added something for 2003 that was non RFC compliant) What you have done so far I deem safe from an AD point of view, but I dont know if your 2003 installer will tolerate this. It should since you are allowed to use the installer after 2007 are installer to let's say extend a cluster etc. My overall judgment is that you are in good shape and you should have no issues continuing down the path you are headed towards I would be interested to see what EXBPA would say about your environment now when you have removed 07 I'm still open to any other comments confirming or debunking these thoughts:

BTW - this has been running for a while now and seems to be fine...Also - you may consider backing up more than PSTs... like the rest of the config...here you'll see there are config files and xml files you may want later...Mike Crowley: MCT, MCSE, MCTS, MCITP: Enterprise Administrator / Messaging Administrator