It seems that many people were able to see the interface to upgrade their characters after US servers did boot up after the maintenance. The instant boost to level 90 will cost 60$ according to the temporary visible character upgrade interface. However, it must be mentioned that the level boost is a poor decision by Blizzard.
World of Warcraft was once a very great game where people could go through adventures, where people had a challenge, where people achieved their goals in the game together. World of Warcaft became soft-boiled after Burning Crusade. The game was still ok with Wrath of the Lich King but they heavily simplified the game later.
Now we are here, the point is reached where you can even buy character upgrades and level boost. The hardcore players left the game, many lost interest in the game and now there is only a crumbly community left which demands more and more simplifications, which does not even revolt against level boost and character upgrades. The framework is now so friable that the free fall of a once perfect game can go on at least from veteran point of view.
What we have seen is a migration period. They simplified the game and adjusted it to a lazy, spoilt and dull community which does not want to have a challenge but reaching everything without doing anything for it. Welcome to World of Warcraft the game full of warlords which pay to win. Warlords of Draenor!