i am in the process of migrating from plesk/FC4 to ISPC3/Lenny and this is my written down notes... maybe this will help you guys too if you are migrating...
creating domain etc at the new server
copying /var/www/vhosts/domain/httpdocs/ to /var/www/domain/web
adjusting permissions where needed
also grepping for old /var/www/vhosts and editing anything to see new path
locate and backup EACH db with phpmyadmin. export then edit the sql file to match new system. then import in with phpmyadmin
emails... i'm right here now..
i've warned users to either 1) move all important mail to a local client, like Thunderbird or Outlook (for those using imap) and that their password would be set to something (unsafe, but they can change it in squirrelmail ie the change_sqlpass plugin) ... or 2) force access into their account, link it to outlook/thunderbird on my end, copy move em, log out, switcharo o to new server, reload their account, change password and setup an webpage telling them their password was reset. (probably the worst method I can think of, but it works...)
O_O i'll let you know how it goes,but i have around 40 accounts to move *sigh* fyi, not very secure, but hey, you ARE the webmaster... you just can't be peeking into their emails... there are methods to migrade qmail to postfix, but because my server's small, this seems easier...