Theres just one but in that. If you lose shinigami powers once, they pretty much are gone for good, unless you of course get new ones transferred to you from another shinigami. Ichigo became a shinigami again only because he managed to find his ~own~ powers, as the ones he got from Rukia were destroyed by Byakua. And how would they lose that spirit energy anyway from being scared?

; I mean, of course it could go down some, but I doubt they'd actually lose it..