PlayStation officially prevents stacking PS+, adds new rules for existing subscribers
PlayStation is cracking down on how PlayStation Plus and PlayStation Now works for existing subscribers. Those hoping to stock up and stack their subscription have found that Sony is preventing them from doing so--and now they're taking even further measures to stop that. If you have a pre-existing subscription to either PlayStation Plus or PlayStation Now, you can't extend it, even if you purchase prepaid cards from retailers.
However, starting in June, they'll begin allowing subscribers to redeem their prepaid cards again, with a major caveat; if you want PlayStation Plus Premium--the highest-paid tier--it won't give you an equal amount of months as to what you would normally get. For example, if you redeem a PlayStation Now 12 (365 days) month prepaid card on PSN, you will only get 6 months (183) days of service of PlayStation Plus Premium. If you want the full 365 days of service, you'll have to downgrade to the lowest tier, PlayStation Plus Essential.
The full conversion chart, provided by Sony, is shown below, which is their attempt at trying to clear up confusion regarding the whole convoluted affair. The updated FAQ featuring the graph can be seen at the very bottom of the official PS Plus page.