We recently used PP to upgrade an NH coded segment on an NH ticket on NH metal. We did this for two segments. While each showed that the upgrades cleared a couple of days before the flights, we were not able to do any form of electronic check-in. Furthermore, at the ticket cunter we were told -- both times -- that there was no upgrade and that this was an economy ticket. After pushback (including showing screenshots of the NH site showing the upgrade) they completed check-in with J seats. So, it works but it was not easy.