Well they already explain how it will scale down. I assume that in the case of equipping an ability from a lower leveled class onto your higher leveled class(IE lvl.4 Cure on your lvl.50 GLD), the ability will probably remain as if it were at lvl.4 effectiveness, being as how its base class(Conjurer) is still just at lvl.4. Were you to raise Conjurer to higher levels, Cure would eventually become more effective even after cross-classing.