removing exchange 2003 at end of migration from sbs 2003 to sbs 2011

I am near the end of an sbs 2003 to sbs 2011 migration following the standard Microsoft documentation.  I only have 5 days left (of the 21 you are allowed to have source and destination server both on the domain for). I have reached the point where I have to remove Exchange Server 2003 from the source server before demoting it and removing it from the domain.  The instructions say that I have to disconnect mail-enabled users from Exchange using AD Users and Computers, but when I did this (cautiously) with one not-so-important user account I lost the mailbox on the destination server also!!  I can't do this with the other mailboxes. I'll get lynched. So, will I be able to remove Exchange 2003 without damaging the new Exchange 2010 setup which I've just got to.  Or should I just try demoting and removing the 2003 server without removing Exchange 2003 first.  I am worried and running out of time. Desparately looking for advice here. Charlie Russel and Sharon Crawford's book doesn't even mention this step, but according to the MS Word doc it is mandatory.