Exchange 2003 to 2010 transition question

I have this small simple Exchange setup;  just a single forest, domain, namespace, and Exchange server.  (Other servers that are DC's though.)  Current (2003) Exchange server is a DC, new (2010) one is not.

Now at 2003, going to 2010.

Servers all upgraded to 2008 R2.  Have installed Ex 2010 on a member (not DC) server.  Have run all the prep stuff I can find that needs to be done, such as PrepareLegacyExchangePermissions, PrepareSchema, PrepareAD.  Everything is at the right SP level and function level and in native mode.

I think that all I need to do now is migrate mailboxes (which I haven't started to learn how to do yet) and when that is done turn off my 2003 Exchange server and change the IP address of my new 2010 server to the same one as the 2003 server, so as to not have to wait while a new address spreads throughout the world of DNS servers.  (By the way, when mailboxes are "moved" do they still exist on the source machine?)

I'm worried that I might be missing something, and especially worried about taking one of those steps that is irreversible and which could leave me with a 2010 server not yet functional and a 2003 server that can't function anymore.

Any and all cautions, suggestions, "have you done this or that" type questions, and other general handholding most welcome.

--Ron Hicks, Washington DC
Ronald HicksAsked:
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I would recommend that you read through this guide:

It is put out by Micrsoft and can be customized to fit the details of your specific migration.  It tells you how to migrate the mailboxes and decommission the old server, as well as anything else you would need to know.  This is what I used for our Exchange migration and I found it very helpful.

Once a mailbox is migrated is only exists on the new server.
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