Physical cards, like at Walmart or GameStop, will have an existing inventory at the old price. This is the same thing that recently happened to Canada with an increase in game-time (but no subscription increase). WoW players in CAN were able to buy physical game time cards at the old price for a few weeks before those sold out and then the new stock of cards was at the new price.
The increase to digital purchases of game-time through the Blizzard Store went up yesterday as noted in the Blizzard blue post.
As someone else said, if ($30 for 2 months) breaks your budget compared to 2 x ($15 for 1 month), then maybe you should really consider a free-to-play game. That’s just not healthy to be stressing over such change if it really impacts your ability to pay mandatory bills like rent, utilities, food, and transportation.