Exchange 2010 upgrade backout plan?

As of last night we just updated our AD Schema for the eventual upgrade from Exchange 2003 to 2010.  Once we install the CAS/HUB role (same server), the CAS will take over as the primary front-end, correct?  Obviously, we need to make DNS changes to point to the new VIP and install certs, but that's what should happen.  If the install fails for some reason, is there a good backout plan?

How do I go back to the original settings before trying again?  I guess what I'm asking is what exactly is changing just in case I need to go back.

Thanks!
-Carlton
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liquid101Asked:
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compaqusConnect With a Mentor Commented:
Users will be still using the 2003 even if the 2010 HUB and CAS is working side by side. You don't change client settings until you know 2010 is working.

revert to OWA: you will still have //2003server.domain.com/exchange for them to login and the old exchange server to point their email clients back.

I only did two installations and I had no issues whatsoever. You could wait for a second opinion on this so you'll feel more comfortable about it.

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compaqusCommented:
You cannot install exchange 2007 over 2003, therefore the 2007 will run in coexsistence with your current 2003 installation until you decide that all is running ok and decommission the older one.


http://www.msexchange.org/tutorials/Implementing-Exchange-Server-2007-coexistence-Exchange-Server-2003.html
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compaqusCommented:

sorry dunno why I was thinking 2007.
same thing applies to 2010
http://technet.microsoft.com/en-us/library/dd638130.aspx
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liquid101Author Commented:
I understand the co-existence piece while installing Exchange 2010.  We're running all new VMs for the CAS/HUB roles.

My question is if something happens during the install.  When CAS takes over as the frontend-- let's say that fails; what then?  I will of course try again and troubleshoot, but if time becomes a factor and we need to be back online quickly with the old Exchange 2003 environment, what do I do?  And time can become a factor since we're supporting over 6500 active users.

Can I simply change the DNS entries to point to the old VIP on the Load Balancer?  What changes within the Exchange console would I have to make to make sure we're still routing properly?

Thanks!
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compaqusConnect With a Mentor Commented:
As I understand, the mail flow remains managed by the 2003 until you're done.
When you install the 2010 a routing group is added and appears in exchange 2003 routing groups that connects the new install.
When you fell that everything works, you move the mail flow to the 2010 (remove the smtp connector in 2003).

http://technet.microsoft.com/en-us/library/gg232715.aspx
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liquid101Author Commented:
Yeah, the mailflow is fine, but users will now be using the CAS/HUB (we're using dual roles on the VMs for the front-end) and that will change how they are routed to the legacy boxes, correct?

What happens if something goes wrong?  How do I go back to the original settings?
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liquid101Author Commented:
I'm really not forecasting anything to go wrong.  I haven't seen anything out there to suggest that something catastrophic will happen since the upgrade essentially runs in parallel during coexistence.  I could foresee something much worse if it was a standard upgrade (overwrite).

That being said, I need to assure my employers that we have a strategy.  I'm assuming that we can just redirect traffic via DNS if need be.  Does that even sound plausible?  
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liquid101Author Commented:
That sounds exactly like what I need as a backout plan.  And since the mail flow will stay on 2003 until the front-end is tested, I think we're good on the next stage (CAS install).

Thanks!
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