Sony has confirmed that those that are currently subscribed to PlayStation Now will be upgraded to PlayStation Plus Premium when it launches, but some users are reporting issues with prepaid cards and stacking subscriptions.
Push Square later provided an update with a support email confirming that there is a temporary “deactivated function for PS Plus/ PS Now subscriptions which does not allow for stacking of either membership on already active memberships.”
Sony have reiterated this statement now that we know when the new tiers are launching, but as reported by Push Square, users are facing issues with both prepaid PS+ cards and stacking subscriptions.
The most likely reason for this is that stacking PS Now would mean saving money in the long term.