Say a character took 5 levels in duskblade, then 5 levels in abjurant champion, then 5 in cleric, then 5 in wizard (it’s not a particularly optimized character). They later decide they want to use abjurant champion’s capstone feature on their wizard class, instead of duskblade. However, the prestige class says:
At each level, you gain new spells per day and an increase in caster level (and spells known, if applicable) as if you had also gained a level in an arcane spellcasting class to which you belonged before adding the prestige class level.
This means that in order to “register” their wizard spellcasting with abjurant champion at all, they need to “move” at least one of their wizard levels “in front of” at least one of the abjurant champion levels (ideally all of them, for caster progression). In effect, they want to go from:
duskblade 5/abjurant champion 5/cleric 5/wizard 5
duskblade 5/wizard 1/abjurant champion 5/cleric 5/wizard 4
Now, the PHB II section on class level rebuilding (pp.197-198) says:
Each time your character completes a rebuild quest, you can change a number of levels equal to 1/5 his character level (rounded up) from one class to any other class (or classes).
So, in the example above, how many levels are they “changing”? Are they only changing one level, as the others get “pushed out of the way”, and therefore they only need to do one rebuild quest?
Or are they changing every level from character level 6th up to character level 16th, and thus need to complete several rebuild quests, changing a handful of levels each time in order to slowly move their wizard level further up the chain?
Or are they not changing any class levels, because in the end every class level they have after rebuilding, they also had before, meaning there’s no actual change taking place? And so, all they need to do is reorganize the way they list things on their character sheet and retrain the abjurant champion class features?