Presumably, you could export the extension file and using whatever editor, you could alter the extensions as needed to reflect the re-assigned extension numbers. You should then be able to delete the existing extensions in the system and import the modified file as a replacement. This should then reflect the changes to the extension, but the phones will continue to try and register with their existing settings which may cause some IP blacklist issues until such time as the phones get the proper associations to their new extensions.
However, a few caveats -
1. The operator/receptionist extension cannot be deleted until you have designated another one.
2. You cannot change the length of the extensions. Once a length has been set, the entire system is set to that to include all the system extensions. If this is needed, I believe it necessary to re-install the system and set the extension range accordingly, but then you could import the modified extension list generated above assuming you had already set these to the new lengths.
3. The phones will still carry the association to the old extensions and unless the password, authid and user (registration details) remain the same, these will have issues. Any phone undergoing an account change will need to be re-provisioned or otherwise accommodated to the new extension and in the meantime some phones may end up on the blacklist. Additionally, the new assignment may mean other BLF settings and the like may need to be modified to their new owner as I assume not all phones are provisioned identically. It may be easier to delete the phone from its current extension and then once it appears as unassigned, provision it to the new extension. Any settings that may have been done to the phone that are not contained within the template will not be changed in the phone. So, if you have made some manual changes that are not a function of the template, these will remain and you may need to alter accordingly.
You might be able to import the new file and then do a mass edit to cause the provisioning to take place and then reboot the phones so they will pick up their new identities.
While I have not done this myself, I think that if you approach it in a logical manner and understand the potential ramifications of new extension assignments not immediately matching existing phone assignments, then you can take the steps to perform the desired transition and mitigate the fallout.
Just be certain that you have a good backup before proceeding and then another following a successful implementation.
