They've already looked at shifting to previously chosen forms. Shifting to a particular form will always activate the same section of a particular dragonish's magic, and those sections are evenly spaced in the magic container, one after the other, separated by sections of magic that aren't activated by any known cause.

Choosing a new form activates the whole form slot, right up to where the shift-activated section of the next unused form slot should begin. The mystery of the inactive sections is explained.