This one is tricky.   My company was obtained by a larger company.  Our
environment is A/D 2003 w/Exchange 2003.  Their environment (though much
larger ,) is A/D 2000 and Exchange 2000.  

Question 1 - Is "downgrading" an option for us given that we use
mail-enabled public folders in 2003? So far our findings are yes for the
mailbox store transfers, but no for the public folders.

If it is determined that a downgrade is impossible (or even very difficult
and risky,) then the parent company is willing to begin its own migration to
A/D 2003 w/Exchange 2003.  

Question 2 - If we migrate to an upgraded or new A/D 2003 structure under a
different domain name, what steps will we need to take to properly move the
user accounts, exchange mailboxes, and Public folders to their new servers?  
The other part of the question is how do we do all of this and keep our
existing mail addresses working under a different domain name?    

I know it's complicated question, but any feedback or reference links will
be greatly appreciated.