As mentioned under 1. Class-specific Actions, upper tier actions will be abolished in favor of scaling the potency of the base action according to level. The effectiveness and MP cost of the action will be dependent upon the level of your current class.
Does this quote not already answer the question? If you have a CNJ lvl 10, and you put Cure onto your GLA lvl 50, the spell will be based off of whatever your current class is.

So as long as you're on your GLA, the spell will be based off of the lvl of your GLA, not your CNJ.