Just throwing my 2 cents in...
Seeing as how canon (directly from Rob) states that there are no retconjurers, I'd recommend not allowing casters to be able to take that school at all. http://www.erfworld.com/wiki/index.php/ ... onjuration
If you still want to use the Retconjuration trait and have it stay true to canon, I'd suggest something like the following:
Retconjuration: Character does not have to select any other secondary traits during creation. All costs for spells are paid for as if the caster does not have any primary or secondary traits. At the start of their side's turn, or when directly engaged in combat, a Retconjurer can sacrifice the Retconjuration trait and choose their primary and remaining secondary traits from the list of all magical traits, excluding Retconjuration. ("What are you talking about? I've always been a Mathamancer...")
This gives the flexibility of being able select the most advantageous school for the situation, but limits the power of the caster.