How to update Budget Type in Budget Register Entry header if the status is 'Completed'

The Budget Type in Budget Register Entry header is empty and the this Budget Register Entry's status is 'Completed'. As checked that the Budget Type field is not editable.

1. Can I change this Budget Type field to 'Editable' in the AOT properties?

2. If I update the empty Budget type from backend, any impact to the Budget balance?

3. Can I reverse this Budget Register Entry? How to reverse this Budget Register Entry