how to change department id primary key to identity and do migration to database

Problem

I need to modify class Department to make Department ID identity then make affect by migrations to database sql server 2005 .

What I Try

I changed [DatabaseGenerated(DatabaseGeneratedOption.None)] from None to identity .

then from console write enable-migrations but it give me message

Migrations have already been enabled in project 'University Data'. To overwrite

the existing migrations configuration, use the -Force parameter.I also using

-Force but no any result or changed happened .

my class Department

[Table("Department")]
public partial class Department      { 
public Department()   
{    
}   
[DatabaseGenerated(DatabaseGeneratedOption.None)] 
[Key] 
public int DepartmentID { get; set; } 
public string DepartmentName { get; set; }  

  } 
 }

I have class department inside container Containerclass as following :

public partial class ContainerClass : DbContext     
 { 
public ContainerClass() : base("name=mycon")      
 {   
 }   
public virtual DbSet<Department> Departments { get; set; } 
 } 

Table department already created in database.

update

i write also in console

1- update-database but also nothing changed


2- add-migration Department but also nothing changed column DepartmentID not changed to identity


3- i add migration then write in up() and down()

AlterColumn("dbo.Department", "DepartmentID", c => c.Int(nullable: false, identity: true));
but it not change column to identity