Unable to edit personnel number in bp transaction

Hi experts,

I have a employee which is created by an idoc from another system with number starting from 0H. When i go to this employee in BP transaction, if i go to identification tab and try to edit the personnel number, it is not editable. My requirement is to assign the personnel number of this employee to another employee by removing the personnel number from here. If i try to assign the personnel number of this employee to another employee, it says personnel number already been assigned.

How can i remove the personnel number from this employee and assign the same to another employee?. Usually in bp transaction, the personnel number is editable i see.

Any helps appreciated.

Thanks in advance.