Here's one way to do this:
1. Use the BAT export utility to export all details of your phones to a CSV file.
2. Modify the extensions in the CSV.
3. Delete all existing phones.
4. Use BAT to import the updated CSV file.
Be sure to make a copy of the original exported CSV file before modifying. It would also be a good idea to test with a couple of phones before updating all devices.
Hope this helps.
Brandon