upgrade.php

<?php
function coauthors_plus_upgrade( $from ) {
	// TODO: handle upgrade failures

	if ( $from < 2.0 ) {
		coauthors_plus_upgrade_20();
	}
}

/**
 * Upgrade to 2.0
 * Updates coauthors from old meta-based storage to taxonomy-based
 */
function coauthors_plus_upgrade_20() {
	global $coauthors_plus;

	// Get all posts with meta_key _coauthor
	$all_posts = get_posts( array( 'numberposts' => '-1', 'meta_key' => '_coauthor' ) );

	if ( is_array( $all_posts ) ) {
		foreach ( $all_posts as $single_post ) {

			// reset execution time limit
			set_time_limit( 60 );

			// create new array
			$coauthors = array();
			// get author id -- try to use get_profile
			$coauthor = get_user_by( 'id', (int) $single_post->post_author );
			if ( is_object( $coauthor ) ) {
				$coauthors[] = $coauthor->user_login;
			}
			// get coauthors id
			$legacy_coauthors = get_post_meta( $single_post->ID, '_coauthor' );

			if ( is_array( $legacy_coauthors ) ) {
				//echo '<p>Has Legacy coauthors';
				foreach ( $legacy_coauthors as $legacy_coauthor ) {
					$legacy_coauthor_login = get_user_by( 'id', (int) $legacy_coauthor );
					if ( is_object( $legacy_coauthor_login ) && ! in_array( $legacy_coauthor_login->user_login, $coauthors ) ) {
						$coauthors[] = $legacy_coauthor_login->user_login;
					}
				}
			} else {
				// No Legacy coauthors
			}
			$coauthors_plus->add_coauthors( $single_post->ID, $coauthors );

		}
	}
}