The Collaborative International Dictionary of English v.0.48:

changeover \changeover\ n.
   an event that results in a transformation.

   Syn: conversion, transition.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

changeover
    n 1: an event that results in a transformation [syn:
         conversion, transition, changeover]

The Free On-line Dictionary of Computing (30 December 2018):

changeover

    The time when a new system has been tested
   successfully and replaces the old system.

   (2003-11-12)