Mailboxes are created against the Name field, Dial by Name uses the Full Name field unless it is blank, then it uses Name. Changing the extension number won't change the mailbox. As long as the new extension numbers don't conflict with other numbering, you should be good to go. If you have an automated attendant, and Voicemail Pro, you might think about keeping the leading digit active, go to another menu to collect digits, and transfer to the new 3 digit extension from the old 4 digit number (old extension is 4123, new is 123, 4 goes to a menu with a 2 second timeout that returns to the AA, but also collects 3 digits and transfers)