Note, that earlier, especially prior to 1970, "upzoning" commonly meant the opposite: rezoning land to more restrictive and particularly residential uses. Fischel  suggests that this came from the metaphor of the land-use pyramid, with restricted residential use at the top of the pyramid of uses.
Seattle Mayor Ed Murray has put forward plans (2017) for upzoning (in this context, allowing for taller and larger buildings) in certain designated zones in the city's downtown, South Lake Union and University districts as part of his Mandatory Housing Affordability (MHA) program. The taller, larger buildings (and connected tear-downs) would be allowed for areas supported by good transit and designated as "urban centers" and "urban villages." It would not touch single-family neighborhoods. The upzoning would go hand in hand with rent-regulated units being included in the new developments or developers' fees paid towards building affordable housing.
In Austin, Texas the conversation around "upzoning," particularly for opponents of the CodeNEXT land development codes of residential structures and developments that could take place while maintaining a "standard neighborhood look."
New York City (The Bronx)
With plans for a Metro-North station on the horizon for Parkchester, Bronx (2017), Bronx Borough President Ruben Diaz, Jr. has spoken of "upzoning" Parkchester and two other Bronx neighborhoods slated for train stations.
“I think that we would be remiss if we didn’t look at the area as a potential way where we can increase density, residential units as well as commercial and real estate spaces. These areas are just ripe for that,” Diaz said.
The tenure of Mayor Michael Bloomberg (2002-2013) saw a lot of upzoning in New York City. That has continued under Mayor Bill DeBlasio (took office in 2014) who has also put a focus on affordable housing requirements and inclusionary zoning.
The Bronx Develops: A Metro-North Station in a City Within a City - Norwood News - 14 September 2017
Homeowners Stand to Gain in Areas to be Rezoned, If They Can Keep Their Homes - Gotham Gazette - 29 April 2017