I don't think that you can change it with out deleting it and recreating it.
Here's what I have done as a work around.
Take over the users mailbox (bind it to yours). Create temporary folders that mirror all of theirs, for example John_Doe_Inbox_Temp, John_Doe_Sent_Items_Temp, John_Doe_Temp, John_Doe_Drafts_Temp, etc. Copy their contents into each respective folder. Export their personal address book.
Once you have a copy of all of their data delete the mailbox. Remove the binding between their mailbox and yours. Delete the users mailbox allowing time for it to replicate to all servers. An hour is safe. Recreate the mailbox with the correct Directory name. Take it over and re-populate it with the contents stored in the Temp folders. Once again release the binding. Delete the temporary folders. Test it with their logon and user profile.