Ah yellowing, you said fading and I was confused as to how a white case could really fade.
I think yellowing happens due to exposure to the sun, over a long time. I have only really seen yellowed HHKB and HHKB Pro1 (Which are both quite old) so I would assume this would take quite some time, especially if your keyboard isn't in the sun at all times. I think ABS will yellow while PBT will not yellow, so as long as the case material has not changed, the pro 2 will eventually yellow.