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

Code:

---

[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 :

Code:

---

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

---

Table department already created in database.

update

i write also in console

update-database but also nothing changed

update2

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

update3

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

Code:

---

AlterColumn("dbo.Department", "DepartmentID", c => c.Int(nullable: false, identity: true));

---

but it not change column to identity

Please write did you decide this problem? If yes, write details. Thank you.