Weird mana changes explanation - A lot of people are confused by the weird mana changes, the reason why spells still require the old mana cost to be cast properly is because of how spell hotfixes work. The mana cost of a spell is built-in your client to prevent an insane amount of requests asking the server "hey, how much mana should I use for that spell?" whenever you spam all your keys.
It makes the server happy, but it means that Blizzard has no way to bypass that check after a spell is hotfixed, you will just have to wait for Patch 4.0.6 to be deployed on live servers to see that change reflected on your client. It doesn't matter anyway, in the end the spell will use the reduced amount of mana and will make you more awesome.
То есть стоимость спелла записана в клиенте игрока. Иначе сервера повешаются от бесконечных запросов. И хотфиксы это дело не меняют. Только в с выходом 4.0.6 все изменения вступят в силу.