Old buildings have to be demolished and new ones built, which may not be in the range of the former tenants. Old residents have to leave either way, which is both the problem itself and the root of other ones.

Anyway, it's the mutation of a city into another one, there are too many variables. Higher density sounds appealing but it may bring other problems, and the old residents end up displaced anyway. It's not going to be pretty, and the result is not going to be the San Francisco everyone loves, but well... it's not my city so why do I care.

Edit: quite a few edits