You can do this one of 2 ways.
A. On the old server, find EXMERGE in the exchange bin directory, or you may have to download and install it (free) from Microsoft. When you launch exmerge, it will run a wizard to select which mailboxes to extract; it extracts each mailbox to an individual .pst
B. If you have a Outlook installed on a test workstation that you can attach to the old server (or install Outlook on the old server), attach Outlook to each user's mailbox and use the Outlook export functionality to export the mailbox to a .pst.
Then from either method above, import the respective .pst into each user's current Outlook profile/.pst.
If there are many users, option A is best since you run the wizard once and it will export all of the mailboxes to individual .pst's; just set the wizard and go away and come back to a directory full of .pst files.