Common sense would say it's a buff, but there are of course edge cases like the guy who said it messed up his odd deck.
For these edge cases I'd argue blizzard should always give full refund if card changed in any way, but we know that won't happen.
Is it really that much of and edge case if the big demons you want to play happened to be odd as well?
In all honesty though, I think Genn and Baku were a mistake. The concept of odd and even decks is cool, but now it adds even more hoops for balancing. So much so, they outright HOF'd them far before they were supposed to rotate out.
But seeing as Wild is still getting balance considerations (the Darkglare Nerf is the most recent evidence for this) they're going to undoubtedly be confronted with an imbalanced Odd/Even deck down the line.
I feel like the concept became a lot more balance restricting than anyone could have guessed.
You should probably be able to disenchant it for a refund, but it's definitely overall a buff. Playing the card a whole turn earlier, or being able to hero power or nether breath or whatever the same turn is a huge buff. You're not playing her for her stats.
A card like Willow which pulls multiple stats worth of minions onto the board gains a lot more from costing 1 mana less than it loses from having -2/-2. You're either being disingenuous for the sake of dust or just a moron if you think this is a "nerf".
1 mana is equivalent to 2/2 of stats, as per the vanilla 1 mana minion with 2/2.
Therefore lowering a minion's cost by 1 mana, along with it's stats by 2/2 is in fact neither a buff nor a nerf. As such, Blizzard usually only offers dust refunds for cards that have been nerfed (and not those changed for no value difference or buffed). This is a precedent that has been set in the past and there's no reason to see any difference here.
Here is why it's a buff... You get to play the card sooner for the tempo play which out weights the stats decreasing. No one plays Willow for the stats alone, they build a deck around this card. Making this card 8 mana cost will make it less awkward to play when you have 10 mana as you can now hero power or play 1 or 2 mana cost cards in the same turn.