Yes, there will be unhappiness, from someone or another, because there are different levels of customers. So instead of generalizing, it's productive to look at the impact on each kind.
Good question. How many codes have been saved up? The game's working much better now than the previous version, so arguably some of those people should already be redeeming. But for those waiting for tournaments, well yeah, those aren't ready yet. Let's say it's ready in 3 months. What are they going to do?
Well, they still need the old cards, so a good number of unredeemed codes are still going to be used to redeem booster credits to purchase older packs, so they can get the staples and good Pokémon that are in the format now. And if these people have been accumulating and saving up codes along the way, then they are probably going to acquire even more codes from Plasma Storm. So they will have to use those and those alone to redeem for packs of Plasma Storm.
So what's the difference? Let's say after redeeming their 18 - 72 codes of Plasma Storm (assuming they bought between 1/2 box and 2 boxes), they won't be able to use their older codes for Plasma Storm.
Is this a fair and reasonable situation to place some of their customers in? The Pokémon Company thinks so. The impact just isn't that big for this type of customer.
I do like your Christmas shopping example, but since gift certificates are like cash (with state-by-state laws that govern their redemption), I think the analogy would be more accurate if was a promotion code to get a significant discount off a certain type of clothing. And yes, maybe that Christmas promotion suddenly doesn't apply to the new line of clothing coming out in the spring. People still need other clothes besides those new spring fashions, so the promotion still has the same value as it did before. It's just not as good as the new promotions also coming out in the spring. To take advantage of that new promotion, yeah, you gotta buy something again.