The term Change of Status (COS) is the name given to the process through which a person can change from one temporary status (nonimmigrant) to another temporary status without having to leave the United States. Typical examples of this process are changing from a student visa status to H-1B. Usually, a successful COS is evidenced by the USCIS issuing you a new I-94.
Changing status from a temporary visa to a green card within the United States is also a change of status, but it is technically referred to as Adjustment of Status (AOS).
The US State Department has made the following announcement today regarding the phased resumption of visa services.