user_capability_migrator

<?php

function balkan_update_user_role( $user_id, $role ) {

	// Site 1
	// Change value if needed
	$prefix_1 = 'main_';

	// Site 2 prefix
	// Change value if needed
	$prefix_2 = 'child_';

	$caps = get_user_meta( $user_id, $prefix_1 . 'capabilities', true );
	$level = get_user_meta( $user_id, $prefix_1 . 'user_level', true );

	if ( $caps ){
		update_user_meta( $user_id, $prefix_2 . 'capabilities', $caps );
	}

	if ( $level ){
		update_user_meta( $user_id, $prefix_2 . 'user_level', $level );
	}
}

add_action( 'set_user_role', 'balkan_update_user_role', 10, 2 );