So some of the dissenting opinions have shown up here. So far I've seen:
"It's too generous to give a refund for a card that's nerfed."
"If you know a patch is coming in a month, then you should know the card is going to be nerfed and accept the loss."
"If this system gets implemented, people will just save up cards to mill them."
SUMMARILY:
It is in no sense of the word "generous" of a company to change something that you purchased with the expectancy of it working as-is without some form of compensation. It is, in-fact, a bait and switch. That may not be the intention, but in all practicality, it is identical.
This is patently false because we have no idea whether they're going to nerf cards or not. There's a lot of outcry against RNR and Drought — but are these people just running the biggest, greediest, deck they can without adding in any cards to deal with these meta boogeymen? For all you know, these cards will not be nerfed at all. This creates uncertainty. Uncertainty is bad for business, and certainly bad for starting players.
Here's a novel idea: rather than changing cards, how about create new cards that interact or provide counterplay with problematic cards like a printed ccg would? I might be old school, but I believe that it's far more interesting to broaden the card pool to deal with various threats rather than to just bring down the nerf hammer on a card. Nerfing a card should be a last resort — only utilized on cards that are completely meta warping that do not have any clear counter or way to interact with. If you don't change 50 freaking cards a patch you don't have to worry about everyone cashing in their collection.
There is no such thing as a perfectly balanced game. Period. That is an illusion. You can come close to balance, but never achieve it. I do not expect cards to be perfect when they come out. There can be unforeseen interactions in the future, or just an oversight that makes it overpowered.
You cannot punish players in a competitive game for utilizing strong cards. They should not be punished for using them. The card is available to everyone to craft.
To summarize, Gwent has really blown away my expectations in terms of design. They've done a great job of making all cards interesting. There are no vanilla cards. There's a great deal of interaction in quite a few archetypes.
However, there is no real problem with refunding cards that are nerfed. People coming from other games will expect it. It's a great service that builds trust between developer and player.
This idea that Hearthstone only does it because they have the money is ridiculous. Is Blizzard wealthy? Yes. But this is CDProjekt. Do you have any idea how much The Witcher 3 grossed? Not to mention they own a retail platform that's almost the only show in town when it comes to old games. They're not impoverished by any stretch of the imagination.
Obviously the game is in beta and things are going to change. With that in mind, I am talking explicitly about having this feature in full release — after most of the balancing should be done already.