In Dragon Quest 9, what's "revocation"?
How does it affect new treasure maps?
Is it the same thing as changing jobs at Alltrades Abbey?
Is there a limit to the number of times each party member can do it?
Revocation is resetting your class from level 99 to level 1. You keep all your skills, but not your spells, nor the stat boosts from your levels (however your stat boosts from passive skills remain). Revocation is required at least once to master every skill in the game, and probably more than once unless you plan optimally.
Revocation nets you a unique item for each class, so if you want 100% item completion, you will need to do it.
The number of times your main character has revocated affects the quality of treasure maps you receive. The more times you have revocated, the higher your expected map quality is, up to a maximum of 10.
Revocation is done when you reach level 99. When you "revocate", you return to level 1 for that job, resetting all your attributes, but not your earned skill points/abilities.
I believe you can do it multiple times per character, though it would be lengthy to do so.