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',
		)
	);

	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 ) ) {
			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;
				}
			}
		}
		$coauthors_plus->add_coauthors( $single_post->ID, $coauthors );
	}
}