Yeah, that would work. At level 9, retrain 5B -> 5C for:
Level 1: 1A, 1B
Level 5: 5A, 5C
Level 9: 9A, 5B
Then at level 10, take expanded spellbook and retrain 1A -> 1C for:
Level 1: 1B, 1C, 1D
Level 5: 5A, 5C, 1A
Level 9: 9A, 5B, 9B
At level 11, drop expanded spellbook and get to the...