No, no new price baseline. If the same tenants move back into the replaced unit, they continue at the rate as if their lease had been continuous (plus rent control raise limits)
That's a pretty good deal. Almost as if having gained tenancy in a rent-controlled property is a (non-transferable) asset worth more than the property itself.
Yes, that's exactly why "tenant buyouts" are a thing in San Francisco. They've created a property right for renters, as well as weird concepts like the "master tenant".
Is the new apartment guaranteed to be the same size as before?