Glad it's coming but a bit surprised the nerf targets cost rather than power.
The problem with the increased cost is that it messes up the interplay with cards that allow you to get 5 and 7 cost cards, respectively, from the tavern. So in that sense it's a pretty strong nerf.
Personally I would have opted for a power nerf, reducing the power you get from both cards each by one.
Targeting the cost will succeed in lowering the win rate, but it will not fix the other inherent problem with those two cards, which is that you could get lucky getting them early and then with a few lucky draws snowball your game to victory without allowing for much counter play by the opponent. And this incidentally feels bad for the opponent because they have little interaction with the game.
With these changes you will get the cards more rarely, but when you do, you can still potentially snowball to victory.
A power nerf would smooth the effect the cards have on the game, making them less pivotal and the snowballing less dramatic. This would deal with the root of the problem and there would be no need for a cost increase that is much harder to balance.