A change key is a key on the lowest level of a master keying system. Change keys are also referred to as day keys. Typically change keys are issued to personnel that require access to one or two areas in a facility. By definition, a change key is not a master key.

Change key can also refer to the specialized keys used to change the combination setting of a combination lock

Combination lock

A combination lock is a type of lock in which a sequence of numbers or symbols is used to open the lock. The sequence may be entered using a single rotating dial which interacts with several discs or cams, by using a set of several rotating discs with inscribed numerals which directly interact with...

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Change key can refer to a special key on the "Sabre" Keyboard, located to the right of the "P" key, and can also be typed by doing ALT+0164, or using '@' in Turbo Sabre.
Sabre

Sabre (computer system)

Sabre Global Distribution System , owned by Sabre Holdings, is used by more than 55,000 travel agencies around the world with more than 400 airlines, 88,000 hotels, 24 car rental brands, and 13 cruise lines...

 is a Global Distribution System (GDS)

Computer reservations system

A computer reservations system is a computerized system used to store and retrieve information and conduct transactions related to air travel. Originally designed and operated by airlines, CRSes were later extended for the use of travel agencies...

, used by travel agencies to book flights, hotel reservations, car rentals, etc.