Human characters may multiclass using the rules in the Advanced Edition Companion. Note that in The Bladed Earth, demi-human characters cannot multiclass.
Human and demi-human characters also have the option of switching classes. To do this, a character must meet all the requirements of the new class and must declare the intention to switch classes when reaching a new level of experience. At that point, the character does not progress in the original class - he is regarded as being "in training" for the new class. The character must still accrue enough XP to reach the next level in its original class, at which time he begins the new class at 1st level, gaining all the advantages of the new class.
For example, Farkas, a human fighter, accrues 8,125 XP, which would make him 4th level in that class. Instead of advancing to 4th level in the fighter class, his player announces that Farkas will switch his class to magic-user. He remains a 3rd level fighter (with all the abilities of such) until he reaches 16,251 XP. At that point, he becomes a 1st level magic-user, adding that class' advantages to his fighter abilities.
As when multiclassing, the saving throws and attack values of a character that switches classes are equal to the best values available for all of the character's classes. Likewise, the notes on conflicting abilities from the Advanced Edition Companion's multiclassing rules apply to characters that switch classes.
A character cannot go back to advancing in a previous class after switching classes.
Demi-human characters cannot switch classes until reaching the maximum level available to them in their current class. (For training purposes, assume that race-classes, such as "elf", reach the next level at half again as much XP as it took to reach their maximum level.)
For example, Goldfinch, whose class is elf, accrues 600,001 XP and reaches 9th level, the maximum for that race-class. He would have to train until he reached 900,001 XP to switch classes.
Demi-human characters cannot switch to classes forbidden to their race.
In all cases of multiclassing and switching classes, the Labyrinth Lord has final say on which class combinations are allowed.