Migrating existing custom field values into a meta box created with the WordPress WP Alchemy Meta Box Class

meta-setup.php

<?php
include_once( plugin_dir_path( __FILE__ ) . 'classes/MetaBox.php' );
include_once( plugin_dir_path( __FILE__ ) . 'classes/MediaAccess.php' );

$meta_boxname = new WPAlchemy_MetaBox( array(
	'id' => '_boxname_meta',
	'title' => 'Metabox Name',
	'template' => plugin_dir_path( __FILE__ ) . 'meta-template.php',
	'mode' => WPALCHEMY_MODE_EXTRACT,
) );

meta-template.php

<table class="form-table custom-meta">
	<thead>
		<tr>
			<th colspan="2">Meta box description goes here.</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<th scope="row">Field Label</th>
			<td>
				<?php 
				$mb->the_field('field_name'); 
				
				// Get value from an existing custom field
				$custom_field = $mb->prefix . str_replace( array($mb->id . '[', ']'), '', $mb->get_the_name() );
				$value = get_post_meta( $mb->current_post_id, $custom_field, TRUE );
				?>
				<input type="text" name="<?php $mb->the_name(); ?>" value="<?php echo $value; ?>" />

			</td>
		</tr>
	</tbody>
</table>