If the WWE Championship design doesn't change during Punk's reign, I believe it'll change if Rock wins the title at the Royal Rumble.
It's possible that the title's design could still change with Punk as champion. If Punk successfully defends the title at Survivor Series next week, the following Raw will mark 364 days with Punk as champion. Even though it'll be one day shy of a year, Punk might unveil a new championship design to celebrate the 1 year milestone. It's also possible that they could wait until the next Raw, after Punk's reign has officially passed the 1 year mark, to unveil a new championship.
If WWE does want Cena vs. Rock II to be for the WWE Championship, then Rock unveiling a new championship design to replace Cena's design could be viewed as another little slap in the face of Cena from The Rock.
Whether Punk or Rock, I believe we're ultimately approaching the final days of the spinner design for the WWE Championship.
If The Rock happened to be the first holder of the new model of WWE title, it might also bring back some legitimacy to the title for all the attitude era fanatics.
The AE marks will only be satisfied if WWE ever returns to an Attitude Era style format. Even if it happened, they'd still find something to gripe about. Personally, I'd rather The Rock didn't win the WWE Championship because of the simple fact that he doesn't need it. Having Cena vs. Rock II for the title, in my opinion, won't jack up interest in the match. If anything, I think it might hurt interest because it pretty much gives away the ending of the match before it's even officially announced. Besides that, it'll mean a couple of months in which the WWE Championship isn't defended at house shows. Given Rock's hectic schedule, including the fact that he's due to start filming a new Hercules movie next year, there might be periods of time in which the WWE Championship doesn't appear on television either. I just don't see The Rock having the title as being at all necessary.