Cisco Unity Tools Depot --> Buld Edit --> Subscribers
Select the extension rang from the drop down menu in the "Select Subscribers", in the following page select the extension tab. You may need to play around a little bit. The good thing is there's a "Test" option which allows you to test the changes before saving them.
MK is correct, but if you are changing to something completely different, it will not work.
For example, if their current mailbox is 4000, and you wanted to change the extension to 3000, then you could do this in Bulk Edit. But if you wanted to change 4000 to 3001 and then another subscriber from 4001 to 3400, there is no real pattern the bulk edit tool can pick up or use to edit and prepend, append, etc.
You can, with caution, go into the SQL tables and look at all the subscribers at once and their numbers. Edit at the table level, rather than through the web page.