> could it be easier to shift commercial to high density residential?
In some places it's possible, as long as the commercial spaces naturally lend themselves to residential occupation, or in places with enough demand to justify the costs of tearing down existing commercial spaces and building residential structures.
But most unused (therefore in theory available) American commercial space isn't comprised of smaller 19th century brick warehouses that you can convert into chic lofts.
Rather, it's more like abandoned or underutilized suburban shopping malls that can't be easily repurposed into housing. The most basic issue is that these buildings don't have many exterior windows. After all, in an indoor shopping mall, the whole point generally is to create an environment that isolates you from the outdoors by keeping it at a large distance.
Apartment buildings need to keep more access to the outdoors, both for light and ventilation reasons.