Replace Add to Cart with Download

function ck_edd_user_download_button( $purchase_form, $args ) {

	global $edd_options;

	if ( !is_user_logged_in() ) {

		return $purchase_form;

	}

	$download_id = (string)$args['download_id'];

	$current_user_id = get_current_user_id();

	// If the user has purchased this item, itterate through their purchases to get the specific

	// purchase data and pull out the key and email associated with it. This is necessary for the

	// generation of the download link

	if ( edd_has_user_purchased( $current_user_id, $download_id, $variable_price_id = null ) ) {

		$user_purchases = edd_get_users_purchases( $current_user_id, -1, false, 'complete' );

		foreach ( $user_purchases as $purchase ) {

			$cart_items    = edd_get_payment_meta_cart_details( $purchase->ID );

			$item_ids = wp_list_pluck( $cart_items, 'id' );

			if ( in_array( $download_id, $item_ids ) ) {

				$email       = edd_get_payment_user_email( $purchase->ID );

				$payment_key = edd_get_payment_key( $purchase->ID );

			}

		}

		$download_ids = array();

		if ( edd_is_bundled_product( $download_id ) ) {

			$download_ids = edd_get_bundled_products( $download_id );

		} else {

			$download_ids[] = $download_id;

		}

		// Setup the style and colors associated with the settings

		$style             = isset( $edd_options['button_style'] )   ? $edd_options['button_style'] : 'button';

		$color             = isset( $edd_options['checkout_color'] ) ? $edd_options['checkout_color'] : 'blue';

		$new_purchase_form = '';

		foreach ( $download_ids as $item ) {

	 		// Attempt to get the file data associated with this download

			$download_data = edd_get_download_files( $item, null );

			if ( $download_data ) {

				foreach ( $download_data as $filekey => $file ) {

	 				// Generate the file URL and then make a link to it

					$file_url = edd_get_download_file_url( $payment_key, $email, $filekey, $item, null );

					$new_purchase_form .= '<a href="' . $file_url . '" class="' . $style . ' ' . $color . ' edd-submit"><span class="edd-add-to-cart-label">Download ' . $file['name'] . '</span></a>&nbsp;';

				}

			}

			// As long as we ended up with links to show, use them.

			if ( !empty( $new_purchase_form ) ) {

				$purchase_form = '<h4>' . __( 'You already own this product. Download it now:', 'edd' ) . '</h4>' . $new_purchase_form;

			}

		}

	}

	return $purchase_form;

}

add_filter( 'edd_purchase_download_form', 'ck_edd_user_download_button', 10, 2 );