It is hard to see what economic, social, or political pressures will push this kind of re-zoning. If much of the population in the single -family zoned neighbourhoods are people who are "all-right" (e.g. boomers - often on public pensions, economic immigrants, and double income professional families), then these people do not need, or even want, the city to be expanding demographically, economically, or even culturally. Given this, it may take half a generation for change to happen, and it will happen when the current home owners, or their heirs, start to want to sell.