You only have to start over if you're not going to have communal bathrooms and kitchens.

For the latter, many of these buildings do punch holes for restaurants closer to the ground floor.

The part we don't talk about in zoning and missing middle conversations is that the US eliminated the lowest income housing, which was temporary, single occupancy, with shared resources ie the Saloon, brothel, short term rent, and multi-tenant housing in the 70s.