You may choose to trade in a s or even a t for extra lesser traits. Just in case, here's a quick table to convert down.

Rolled a... Cashes in for... a a s a a t s a t a a a

Keep in mind that qualities bought with s and t are intended to be much more beneficial, so much so that you probably won't want to trade down unless you have something very specific you need. Also remember that extra results don't necessarily need to be spent on the spell itself; just like any other roll, it may confer story benefits.