'page.changeSlug:after' => function ($newPage, $oldPage) {
$newPage->update(['fieldname' => $newPage->slug()]);
}
This should immediately show the new value. I don’t know how the data is stored in your field, you might have to adapt the code, also, since this looks you are using a multilange site, you would have to pass the language code as second parameter if you want to update other languages then the default language (might not be intended in this case).