cutover

An engineering term, "cutover" is the process of transitioning from one system to a a new system. It specifically refers to switching from an old (hardware and/or software) system to a replacement system, covering the overlap from when the new system is live until the old system has been shut down.

Sometimes known as "going live" the cutover is the point at which a new program or system takes over, usually from a previous version, and the old program is no longer used. On major developments, this point is reached when the new software has been written, tested, and runs satisfactorily, in parallel with the old, for an agreed period.