<self promoting spam>
I am starting up a webhost company, and will be offering cPanel based hosting at VERY competative prices.
If your existing accounts are cPanel accounts, you could set your cPbackup to go straight to my server, then I could import your accounts keeping all your cust's settings intact.
I'm also a bit nifty with updating DNS entries to force the worlds DNS servers to provide the new details.
PM me if you have any questions.
</self promoting spam>
cPanel has some nice backup features, including the ability to back up entire accounts WITH all their settings.
This backup can remain on your cPanel box, or be FTP transfered to another server.
If you have full backups of your customers sites, simply change the index page on each homepage to a "DNS updating" notice on the old box, then restore the backups onto a new cPanel box
HTH