Bulk Updating Primary Extension

As part of renumbering a large number of DNs I recently ran into weird issue whilst using the BAT tool to bulk update the primary extension on end users. Having renumbered all the directory numbers using BAT & then trying to update end user's primary extension to match their new number kept failing. The BAT error report showed the error below for each user that it failed to change the primary extension:

19081 Can not change the fkNumPlan field on EndUserNumPlanMap

As both the new & old DNs exist there shouldn't be a problem, updating the primary extension to a non-existent number produces the error message below:

WARNING : PRIMARY EXTENSION : 6701 in Internal does not exist. If all other values in the CSV are correct, then user will be inserted/updated without primary extension.

However it wouldn't update the primary extension until I'd deleted the old DNs, after having done that the BAT update worked just fine. So for CUCM 9.1.2 at least, the order of operations for bulk renumbering is:

1. Use BAT to update phones & their DNs
2. Delete old DNs
3. Use BAT to update user's primary extension