How to Change an Existing Products Attribute Set

I am working on a project which has different type of products having different product fields, initially i have created all the product under same default Attribute set, but after some time i realize that it will be tough to manage the products so i created different set of attribute sets, since i have already inserted lot of products and at such stage changing each products attribute set is something difficult process.

After doing some research i got a code to add the functionality to change a simple products attribute set directly on the Catalog > Manage Products page.

To achieve this go to file app/code/core/Mage/Adminhtml/Block/Catalog/Product/Grid.php
around line 253 add below line of code in function _prepareMassaction():

$sets = Mage::getResourceModel('eav/entity_attribute_set_collection')

->setEntityTypeFilter(Mage::getModel('catalog/product')

->getResource()->getTypeId())->load()->toOptionHash(); 

array_unshift($statuses, array('label'=>", 'value'=>"));

$this->getMassactionBlock()->addItem('attribute_set', array(

    'label'=> Mage::helper('catalog')->__('Change attribute set'),

    'url' => $this->getUrl('*/*/massAttributeSet', array('_current'=>true)),

    'additional' => array(

        'visibility' => array(

            'name' => 'attribute_set',

            'type' => 'select',

            'class' => 'required-entry',

            'label' => Mage::helper('catalog')->__('Attribute Set'),

            'values' => $sets

        )

    )

));

And in this file: app/code/core/Mage/Adminhtml/controllers/Catalog/ProductController.php (anywhere in the class) add a new function:

public function massAttributeSetAction(){ 

    $productIds = $this->getRequest()->getParam('product');

    $storeId = (int)$this->getRequest()->getParam('store', 0);

    if(!is_array($productIds)) {

        $this->_getSession()->addError($this->__('Please select product(s)'));

    } else {

        try {

            foreach ($productIds as $productId) {

                $product = Mage::getSingleton('catalog/product')

                ->unsetData()

                ->setStoreId($storeId)

                ->load($productId)

                ->setAttributeSetId($this->getRequest()->getParam('attribute_set'))

                ->setIsMassupdate(true)

                ->save();

            }

            Mage::dispatchEvent('catalog_product_massupdate_after', array('products'=>$productIds));

            $this->_getSession()->addSuccess($this->__('Total of %d record(s) were successfully updated', count($productIds)));

        } catch (Exception $e) {

            $this->_getSession()->addError($e->getMessage());

        }

    } 

    $this->_redirect('*/*/', array('store'=>(int)$this->getRequest()->getParam('store', 0)));

}

Now go to manage product in admin panel now you will be able to select the product and from action dropdown you can select the attribute set.