Migrating existing email from 10.4 NON-server to 10.6 server, ~beginner

Question

I have been running a domain for many years off a stock Mac OS running plain 10.4, with a few third-party config aids. I want to move the whole domain to 10.6 Server, but my current question focuses on migrating emails. I fully expect to have to manually recreate all the user accounts (alternative ideas are welcome), but I want to maintain the entire email history for these accounts, as-is. (I use IMAP exclusively, so all the files reside on the server.)
In my 10.4 system, inbox emails live in /var/mail, and anything else that's in a mail folder lives in ~/Library/Mail/IMAP/{folders} . Here are some things I don't know:
1) How (if at all) the 10.6 implementation of mail services differs regarding location/format of email files. Also, if there's any difference in the functionality of /etc/aliases.
2) How to transfer the files from old server to the new one. This is a "one box" domain. Since the domain's DNS (and web, ftp, firewall, etc.) is hosted on the same machine, I'm not sure how I could have them live on the internet at the same time (though obviously I can have one machine acting purely as a client on my network). I do have an extra static IP available as well.
3) How I might minimize downtime, and park in the ether any inbound email messages to my domain while I make the transition and debug it.
I'm hesitant to POP the messages off the server into a client, and POP them back on to the new server, for fear of losing the original time stamps (which are crucial), plus it'd be a big hassle. On the other hand, I don't know what havoc would come from trying to replace or splice in my existing email server files onto the new machine.
I have successfully configured a different domain on 10.6 Server, though it was a PITA, and took several attempts. (Not looking forward to another DNS set up. And woe be to those who wish to change the initial IP number on a SLS machine - there are residual references that are unreachable even with the CLI tools.) So I'm not a total noob, but also I'm not a network admin wizard by any stretch.
I'd welcome any helpful advice on this question, or generic strategies for porting an existing domain to 10.6 server. User files, permissions, web servers are pretty straightforward, though I'd also be curious if there's a SLS firewall configuration equivalent (in function and ease of setup) to standard OS X "internet sharing" preference, which I currently use (with two ethernet ports). Thanks so much.

Answer 1

Thanks very much Jeff. The first sentence gets to the nut-meat of the issue (though I leave the question open for any additional comment).
And I appreciate the links to migration tools. My recollection - perhaps wrong - is that the 10.6 migration tools only work for earlier versions of OS X Server, and not for the end-user OS (which is what I have now).
My server has a half-dozen or so user accounts, with a total volume of <10 GB of email files - small. (There are also hundreds of email account aliases, which is why I asked about /etc/aliases functionality.)
The concept of DNS is simple, but I always have a hard time telling if I've screwed up the configuration or a "failure" is just propagation latency of the domain/IP mapping - I don't have an intermediate DNS server/service between the domain registrar and my domain's server, which can make DNS config and testing quite frustrating.
The rationale for NAT/DHCP has to do with internal network architecture and layered security for remote (e.g., VNC) access, but the whole story is not very interesting. I will take your point under advisement, though.