We are currently running Exchange Server 5.5 on a Windows Server 2000 domain controller operating in mixed mode. We are beginning the process of migrating this to Exchange 2003 running on a Windows Server 2003 domain controller and migrating our domain to Active Directory. We really need the entire migration to be performed during a weekend, because any weekday downtime is really disastrous to our clients, who are financial institutions. We have had conversations with a local consultant about helping us with the migration, as our network admin has thrown up her hands and said it is beyond her. The consultant is adamant that the migration cannot be performed in 48 hours, and that we must plan to have our entire system shut down on either a Friday or a Monday -- or possibly both. The network admin agrees with them, saying that, "There is no other company anywhere that uses Exchange the way we do. We have more customization, automation, and leveraging of public folders and other groupware features than anyone else." (That is pretty much a direct quote.)
I am a programmer, not a network admin, and my experience with Exchange is mostly limited to developing custom solutions for the company on it, but I have to say that this doesn't ring true to me. We are a very small company (about 25 employees), and I simply do not believe that our Exchange implementation is somehow more complex than any other company out there. We have a custom COM add-in that automates filing eMail correspondence by Client based on the domain name of the recipient, a public calendar for keeping company-wide schedule information, some public folders with custom forms that facilitate the submission and approval of procedures, some log directories that are posted to by other custom software we've written, a couple of public contact folders that are leveraged by our fax server, and a few other miscellaneous things. But, to me, it is absurd to assert that this somehow constitutes the height of customization and complexity among all the companies running Exchange Server anywhere in the world.
I also find it hard to believe that the migration can't be accomplished in a weekend, given our size, but our network admin refuses to look for another consultant and so we are basically at a standoff. I guess I'm looking for some guidance here from experienced Exchange admins, especially those that may have performed a migration that is somewhat similar. Does anyone think the information we are getting from the consultant and our network admin sounds credible? Has anyone used a consultant for a migration of this nature that they would recommend highly? Are there any other suggestions you can give that would either help me make the case that we should not *ensure* downtime by building it into the plan (of course I realize that things can always go wrong) or help me see the light and get on board with what the consultant and network admin are saying?