Right now, the high cost of housing in the Bay Area is largely due to a couple of factors: 1 - high cost of labor. Rezoning won't change that. 2 - restrictions on right to build. Rezoning will reduce this. 3 - high cost of land. Rezoning will increase this, by making it possible to build more units of housing on a given area of land.
For most current homeowners, it's likely that 3 will outweigh 2. Developers will buy up some portion of single family housing stock (and low-density multifamily stock) to rebuild the lots with denser housing. That process will both increase demand and reduce supply of those types of housing, resulting in price increases.
At the same time, the average unit of housing will cost less, because there will be many more condos/apartments available. Owners of high-density condos would be the most vulnerable to seeing their asset values decrease. However, building big condo buildings is still going to be a complicated endeavor, so price adjustments will happen slowly over time. No one's going to get hosed, although their asset prices might go up more slowly over time than otherwise, or even be eroded by inflation in the long term.
At the same time, many of those condo owners will still come out ahead. They may be able to upgrade to a nicer condo than they otherwise could have afforded. They may also benefit from the second-order effects of lower average housing prices, in the form of reduced costs for non-housing items. If they run a business, it will be easier for them to attract and retain workers. Schools and other public services will also be better able to attract and retain quality staff, at a lower wage bill. Etc.