Second class transfer: the three marks for every class
The second class change at level 40 costs three marks. Each mark is the reward of a long quest, open from levels 35, 37 and 39. This page shows which three quests every class needs and links their walkthroughs.
How the marks are split
Every class needs one quest of each kind. The Trial (level 35) depends on the role: fighters, knights, scouts, wizards, healers and Dwarves each have their own. The Testimony (level 37) depends on the race: Testimony of Trust for Humans, Testimony of Life for Elves, Testimony of Fate for Dark Elves, Testimony of Glory for Orcs and Testimony of Prosperity for Dwarves. The Test (level 39) is what separates the classes of one branch from each other.
With the three marks in the inventory, talk to the Grand Master of your guild at level 40. The marks cannot be traded or dropped, so every character earns them personally.
The same hunting grounds come up again and again: the Plains of the Lizardmen and the Ivory Tower crater near Oren, the marshes of Cruma Tower and the Execution Grounds near Dion, Death Pass and Breka's Stronghold near Giran. Carrying two quests at once saves many trips.
Trial of the Challenger is the one mark made only of boss fights; take a party or come a few levels higher.