If the ISP has IMAP access, what you could do is remove the user/pass for the old POP3-based stuff, add a "new" account that accesses via IMAP, create folders on the IMAP account, and mass-drag stuff from the locally-saved email folders to the new IMAP stuff. In theory, they'll then be able to access everything via the webmail interface.
 Just did this in Thunderbird the other day, it may work with OE too. It was a little slow, but worked automatically.
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