Which of the bunkers are you talking about?
It's been 3-4 years since I was there, but as of my last visit, few of the key bunkers around the greens had been changed ... they had only rebuilt bunkers where they were moving the fairway bunkers, and most of those still stuck out like a sore thumb [#8, #15, #17]. Whether that's on Tom Fazio's crew, or the contractor, is a matter of opinion.
I think a few of the newer bunkers have been rebuilt again so they are more in scale ... which is not what most people would call "aging".
P.S. The "original architectural intent" of Riviera was to have bigger greens and less flashy bunkers. Photos from the 1950's are clear that the bunkers were not as deep or as sexy back then, and that the big lips have built up over many years of sand splash from play. When the club embarked on this restoration project many years ago, they decided they did not want to go back to the true architectural intent, as far as the bunker styling was concerned, so most of this is a moot point.