Add-Migration InitialMigrations -IgnoreChanges
This should generate a blank "InitialMigration" file. Now, add any desired changes to the class you want. Once changes are added, run the update command again:
update-database -verbose
Now the automatic migration will be applied and the table will be altered with your changes.
Edit:
Here is a solution to migrate identity 1 to 2 Upgrading from ASP.NET.Identity 1.0 to 2.0
Use this manual migration
public override void Up()
{
RenameColumn(table: "dbo.AspNetUserClaims", name: "User_Id", newName: "UserId");
RenameIndex(table: "dbo.AspNetUserClaims", name: "IX_User_Id", newName: "IX_UserId");
DropPrimaryKey("dbo.AspNetUserLogins");
AddColumn("dbo.AspNetUsers", "Email", c => c.String(maxLength: 256));
AddColumn("dbo.AspNetUsers", "EmailConfirmed", c => c.Boolean(nullable: false));
AddColumn("dbo.AspNetUsers", "PhoneNumber", c => c.String());
AddColumn("dbo.AspNetUsers", "PhoneNumberConfirmed", c => c.Boolean(nullable: false));
AddColumn("dbo.AspNetUsers", "TwoFactorEnabled", c => c.Boolean(nullable: false));
AddColumn("dbo.AspNetUsers", "LockoutEndDateUtc", c => c.DateTime());
AddColumn("dbo.AspNetUsers", "LockoutEnabled", c => c.Boolean(nullable: false));
AddColumn("dbo.AspNetUsers", "AccessFailedCount", c => c.Int(nullable: false));
AlterColumn("dbo.AspNetUsers", "UserName", c => c.String(nullable: false, maxLength: 256));
AlterColumn("dbo.AspNetUsers", "FirstName", c => c.String(nullable: false));
AlterColumn("dbo.AspNetUsers", "LastName", c => c.String(nullable: false));
AddColumn("dbo.AspNetUsers", "CreatedDateTime", c => c.DateTime(nullable: false));
AlterColumn("dbo.AspNetRoles", "Name", c => c.String(nullable: false, maxLength: 256));
AddPrimaryKey("dbo.AspNetUserLogins", new[] { "LoginProvider", "ProviderKey", "UserId" });
CreateIndex("dbo.AspNetUsers", "UserName", unique: true, name: "UserNameIndex");
CreateIndex("dbo.AspNetRoles", "Name", unique: true, name: "RoleNameIndex");
DropColumn("dbo.AspNetUsers", "Discriminator");
}