Retraining class levels: One at a time or all at once?

[The White Sorcerer]

When you retrain multiple class levels (as per Ultimate Campaign), do the exchanges happen one at a time or all at once?

Let's say I have a barbarian 5 who wants to be a fighter 5. He spends the necessary time and money to retrain all the levels in a row. Does he go through Bbn4/Ftr1, Bbn3/Ftr2, etc. or jump straight from Bbn5 to Ftr5?

I am leaning towards the former, but would like a confirmation from someone more knowledgeable about the rules.

[Volchur]

It would seem to me that each training "session" would give you the level for your fighter, thereby removing it from the barbarian class.

"When you retrain a class level, you lose all the benefits of the highest level you have in that class. You immediately select a different class, add a level in that class, and gain all the benefits of that new class level."

I would allow for it, however, to just go from Bbn5 to Ftr5, just keep in mind that you still have to retrain any of the feats, one per training session, if you did not want the specific feat, or if you lost qualifications for the feat. The same going for attribute changes, etc.

"This retraining does not allow you to reselect the feats your character gains at odd levels or the ability score increases your character gains every four levels (though you can retrain those options separately). If retraining a class level means you no longer qualify for a feat, prestige class, or other ability you have, you can't use that feat, prestige class, or ability until you meet the qualifications again. (You can still retrain that feat, prestige class, or other ability.)"

[blahpers]

If you don't mind me asking, what is the situation that brings this up? Was the training interrupted? Normally this wouldn't make much difference.