I've no real reason other than recent vague dev answers to say this but I suspect a Special Ability raises the relevant qualities by the quivalent of a star when the event occurs to trigger its use. Or something like that.

If it was me I'd stick with the older one and train up the younger one ready to step in when needed - since there didn't appear to be a need to replace him in the first place. But tbh u probably won't break your team either way.