Switching to Native mode from Mixed Exchange 5.5 and 2003 environment

I just finished up on a 'swing upgrade' of my exchange server to 2003.  I started out with a 5.5 server called 'a', and installed a temp server called 'b'.  I moved all of the mailboxes over, and ran like that for a week.  Originally Microsoft stated that if I wanted to keep the server name the same on the final server, I'd have to do it this way.  What I didn't find out until later is that all of the outlook clients automatically updated their profiles to recognize the new server, or I wouldn't have worried about putting up a temp server and migrating everyone!  I was afraid that since the mailserver name changed, I'd have to update every outlook client to point to the new server (2-300 of them!).  Anyway, I decommissioned the original server this weekend, pulled it out of AD, DNS, Wins etc, brought up a new server with the same name as the origninal 'A' server, and moved the mailboxes back over to it.  My problem now is I can't seem to switch over to 'native' mode, the button is grayed out.  I've uninstalled the ADC, and followed the procedures for pulling the last server.  Unfortunately, I didn't see the small link to the article that gives the procedures for pulling the first server out of the group.  First and last server, gets confusing eh?  So my question is how do I get this server/site into native mode?

Thanks!