I think the plastic can still progressively yellow, due to it's exposures (light, chemicals, smoke, humidity, contact with other plastics) before it is protected by the UV cases. It'll slow the process, but I'm thinking that if the plastic has started deteriorating before it is encased, it could well continue to yellow as the plastic bonds continue to break down.
I think AFA grading is only a protective measure if you do it as soon as the toy is released.