Unable to edit Department/Job etc. while trying to Add Assignment

Hi,

We have a requirement where client wants to add temporary assignments to employees. Like adding employee in different department for a 6-8 month time etc.
I was trying to Add assignment (My client group-> show more-> add assignment-> search person) but there it is not allowing me to edit all the details like department/job etc.

Can someone help me with why it is not allowing me to change department or other information while adding assignment.

Thanks,

Jagruti