Cross Forest Migration Exchange 2007 to Exchange 2010

Hello All,
I am doing my first Cross Forest Migration from Exchange 2007 SP3 to Exchange 2010 SP1.  I have DNS and all my trusts in place.
My destination forest has some of the users in it already as we are consolidating into an existing domain.  So I believe my next steps are to run the ADMT to create the users that do NOT aready exist.
1.  Can/Should I run the ADMT again the users that DO already exist to copy over any specific attributes?  It is important that the destination user IDs are not altered in such a was as they cannot be used in their current environment.  Also, I'd like to COPY them and leave the source User IDs intact and functional.  Is this a pipe dream?

Then after I get all the users created:
2. Do I then use the ADMT tool to create the Dist Lists?  Or do I do that after the mailboxes move over?

Then I move the mailboxes using new-moverequest shell command
3.  Because I want to move these and test them prior to "go-live" is it possible to leave the source mailboxes intact and functional?  Basically I only want to COPY the mailboxes, not MOVE them.

I remember with Exchange 2003 to 2010 migrations you had to worry about legacyDNs to avoid NDRs when replying to old messages after a move.  
4.  Do you have to worry about anything like that when going from Exchange 2007-2010 cross forest?  Will my users be able to click on an old internal message, hit reply, and it will go successfully?

I know this is a lot to ask, but I really want to get a level of comfort before proceeding.  I'd give 1000 points for this if I could.

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Did you get a chance to read\review the below blog series:

Exchange 2010 Cross-Forest Migration Step by Step Guide – Part I

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As per your requirement it seems you may need to use 3rd Party Exchange Migration tool E.g. Quest Exchange Migration because when you scheduled users for migration you may want to have roll-back option as well, you may also need to consider this when you are running migration for multiple weeks

martinichickAuthor Commented:
Hi E_aravind.   I did indeed see that blog and have had success migrating users and mailboxes into the new domain.  What I am failing on is moving EXISTING users.  Basically I have some user IDs in Target.local and when I run prepare-moverequest against these accounts, it creates a new, oddly named MEU, instead of altering the existing target user.  I have tried skipping the prepare step, running ADMT to merge a Source account with an Existing Target user, then try to run New-moverequest, but it says that it can't find the target user.  So while your link was helpful, it doesn't fix the existing user issue.  Any reference links for this?  I will give you partial points for this referal.


I'm not sure if you have checked the below link...I would suggest to move all the source users to a specific target OU.... so that when you move them next time it overrides those existing users rather creating a new or if the mailbox is not associated and its just a user object than you can get rid of it

martinichickAuthor Commented:
The references were great, but did not solve my problem completely
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