Add new auto increment column to existing table

My old migration is:

Schema::create('item_tag', function (Blueprint $table) {

    $table->integer('item_id')->unsigned()->index();
    $table->foreign('item_id')->references('id')->on('items')->onDelete('cascade');

    $table->integer('tag_id')->unsigned()->index();
    $table->foreign('tag_id')->references('id')->on('tags')->onDelete('cascade');

    $table->primary(['item_id', 'tag_id']);

});

Now I want add new auto increment column to this and drop old primary key

I try this:

Schema::table('item_tag', function (Blueprint $table) {

    $table->unsignedInteger('id', true)->first();

    $table->dropPrimary();
    $table->primary('id');

});

But I have a error after migrate:

SQLSTATE[42000]: Syntax error or access violation: 1068 Multiple primary key defined

Drop the primary key in a separate migration and remove $table->primary('id'). Adding an AUTO_INCREMENT column automatically creates a primary key:

Schema::table('item_tag', function (Blueprint $table) {   
    $table->dropPrimary();
});

Schema::table('item_tag', function (Blueprint $table) {
    $table->unsignedInteger('id', true)->first();
});

You can also simplify the second migration:

Schema::table('item_tag', function (Blueprint $table) {
    $table->increments('id')->first();
});