Say a wizard is just about to level to 9th, and his current daily spells are:
Level 1: 1A, 1B
Level 5: 5A, 5B
After leveling, he wants to end up with:
Level 1: 1B, 1C
Level 5: 5A, 1A
Level 9: 9A, 5B
That would require two retrainings (5B -> 1A and 1A -> 1C), so it doesn't work. That's...