This topic title is misleading. California law wasn’t changed. From the program start, white/green HOV stickers were issued with January 1, 2019 expirations. Just as the previous yellow HOV stickers expired in 2011. And the current red stickers will expire January 1, 2022.
Also the fuel capacity limits on REx vehicles are to meet BEVx classification under the ZEV credit program that applies to vehicles sold in California, Connecticut, Maine, Maryland, Massachusetts, New Jersey, New York, Oregon, Rhode Island, and Vermont.
The HOV sticker and ZEV programs are independent, which is why REx models got green stickers just like other plug-in hybrids such as the Volt. Green stickers were originally intended to be limited in number while the white ones were unlimited. But the legislature kept increasing the green limit so the state went back to just one color for the third phase.
If you stop to think about it, the HOV sticker expirations make sense because the program exists to encourage the use of new, low emissions vehicles. If the stickers never expired, there would be less incentive for people to help renew the overall fleet.