This gist will provide legacy support for the instance when you need to convert an ACF field from a plain text field to a multi-select taxonomy field. It will convert existing values to the proper array format and pull in and convert original values to array. WARNING: It does not create new terms. This could be added pretty easily tho, however o…

acf-convert-text-to-taxonomy-multi-select.php

<?php

# create your taxonomy
# In this example, I'm using "landing-section-class" as the taxonomy

# Remove the unneeded meta box
add_action( 'admin_menu', '_briand_remove_landing_section_class_metabox' );

function _briand_remove_landing_section_class_metabox() {
	remove_meta_box( 'tagsdiv-landing-section-class', 'page', 'side' );
}

# Handle load_value to convert values to arrays
# All my instances use the field name section_class, however each instance would need to be filtered
add_filter( 'acf/load_value/name=section_class', '_briand_section_class_legacy_support', 9, 3 );

function _briand_section_class_legacy_support( $value, $post_id = NULL, $field = NULL ){
	if( $field['field_type'] != 'multi_select' || ! $field['taxonomy'] )
		return $value;

	// Locate the legacy value
	if( ! $value ){
		$meta_key = _briand_get_meta_key_by_value( $field['key'], $post_id );
		if( $meta_key ){
			$meta_key = substr( $meta_key, 1 );
			$original_value = get_post_meta( $post_id, $meta_key, true );
			if( $original_value && is_string( $original_value ) ){
				$value = $original_value;
			}
		}
	}

	// Either no value or value is already an array
	if( ! $value || ! is_string( $value ) )
		return $value;

	// convert to an array
	$classes = explode( ' ', trim( $value ) );
	$terms = array();
	foreach( $classes as $class ){
		$term = get_term_by( 'name', $class, $field['taxonomy'] );

		if( $term ){
			$terms[] = $term->term_id;
		} // TODO: add support for missing terms (add term)
	}

	if( $terms )
		$value = $terms;

	return $value;
}

function _briand_get_meta_key_by_value( $meta_value, $post_id = NULL ){
	if( ! $post_id )
		$post_id = get_the_ID();

	global $wpdb;
	$sql = $wpdb->prepare( "SELECT `meta_key` FROM {$wpdb->postmeta} WHERE `post_id` = %s AND `meta_value` = %s",
		$post_id,
		$meta_value
	);

	$row = $wpdb->get_row( $sql );

	if( ! $row )
		return false;

	return $row->meta_key;
}