Even though Exchange 2010 is not a finished product yet, it seems kind of strange discussing a transition to it at this stage, but there are some things that you should know if you are planning on upgrading your current Exchange environment to Exchange 2010 in the future.
Active Directory Prerequisites:
First there are requirements on Active Directory. All Domain Controllers must be running at least Windows Server 2003 SP2 in sites where you want to deploy Exchange 2010 servers. In addition, your Forest must be at a Windows 2003 functional level.
You can have Domain Controllers running Windows Server 2008 and even RODC, but Exchange will not use them. Domain Controllers should preferably run 64 bit version of Windows to run smoother and better handle the load from Exchange and other clients.
Your current messaging infrastructure cannot have any earlier release then Exchange 2003. If you do, you must first upgrade/transition to at least Exchange 2003.Exchange 2003 must be least Service Pack 2 and Exchange 2007 servers must also be running Service Pack 2 for Exchange.
So you can meet all the prerequisites by upgrading your Exchange servers to 2003 Service Pack 2 and then upgrading all your Global Catalog servers to Windows Server 2003 Service Pack 2.
Other notable items:
Exchange 2010 only runs on a 64 bit architecture. In fact, unlike Exchange 2007, there is not even a 32 bit demo or lab version.
In addition, Exchange 2010 will only work on Windows Server 2008 or Windows Server 2008 R2.
You must also apply schema updates to the Active Directory.
How to make the transition:
The transition begins with building the Exchange 2010 environment in parallel with your current Exchange environment, starting with your sites that are facing Internet. The reason for starting at the internet facing site is that you must start by, not replacing, but standing up a new CAS server, similar to what you did when introducing Exchange 2007. The big difference here is that Exchange 2007 CAS servers can proxy requests going to Exchange 2000 and 2003 backend servers but Exchange 2010 CAS servers cannot and will only send a redirect back to the client to the old Exchange 2003 Front End or Exchange 2007 CAS depending on where the mailbox is located. This means that you cannot replace your current Front End or CAS server, you must live with both the old and new system together as long as you have mailboxes located on old servers. Another thing you must do is to copy your current certificate to your new Exchange 2010 CAS server and get a new one and place it on the old Front End or CAS server. You will then have, for example, a certificate with the name of “mail.domain.com” on Exchange 2010 CAS and “legacy.domain.com” on the old Front End or CAS server.
When users connects to Exchange 2010 CAS and have authenticated, Exchange will know where the mailbox is located and if it is on the legacy Exchange it will send a redirect to the client to connect to the legacy URL that you configure. If the mailbox is located on Exchange 2010 everything is good and no redirection take place.
To make your life simpler, you should consider consolidating your namespace to only one name otherwise the transition will be more troublesome with more URL and certificates to deploy. Another important consideration to keep in mind is that you will need an extra Internet IP addresses during the transition.
Unified Messaging servers behave the same as CAS servers do, they do a redirect to the old UM server. So make sure that you send the initial SIP communication to Exchange 2010 UM server, and it will redirect if needed. This is true if you don’t have OCS connected to UM, in this case you need to create a new dial plan and assign it to the Exchange 2010 UM server.
An Exchange 2010 HUB server will not talk to an Exchange 2007 mailbox server but will be able to send mail to Exchange 2007 HUB that in turn can communicate with the old mailbox server. To make this work you also need extra HUB server, since the old ones must be around as long as you have legacy mailboxes.
Your existing mailbox servers obviously will have to remain in parallel, since both sets of servers have to be running, to move mailboxes in between them. Legacy mailbox servers can be uninstalled when they don’t have any mailboxes located on them. One of the cool new features in Exchange 2010 is the Online Move Mailbox. This allows administrators to move a mailbox without the user being disconnected during the process until the last minute when all mail have been replicated and Active Directory replication takes place. Online Move Mailbox is possible between Exchange 2010 server and from Exchange 2007 to 2010.
The only server you can replace is the Edge role and this can happen anytime during the transition, as long as you subscribe it to an Exchange 2010 HUB server.Be aware that all of these steps don’t work in the current public beta, but they will work when Exchange 2010 goes RTM sometime later this year. Also please remember as tempting as it might be, you should not put in the current beta in your production environment.