The NIK unregistration process on Live.On numbers will automatically occur if your number has been terminated. Then you can re-use the NIK to activate another number with the same network (XL Axiata network).
This process will occur if you do not renew your package for 4 consecutive months, so your number is considered terminated.
Unregistering NIK is the same as deactivating your Live.On number. For that, make sure the number is not registered and is still active on any account you have because, with this number deactivation, the number will be available for other new customers.