Memory Replacement

Memory Replacement is a psycho-engeneering process, used by the Combine in course of transforming humans to Overwatch Soldiers and Overwatch Elites, also some of the Civil Protection Officers are able to wiilingly undergo such process to advance through CP ranks. This process is generally done through use of bizzare machines, capable of operating memory-cells of human brain without need to expose brain itself

Process
Memory-replacement results in all personal-related memories being wiped out (this DOES NOT mean that personality is totally erased, but all personal-related memory is wiped, so "subject" no longer remember who he was before procedure) and replaced with combat training programming and Combine indoctrination This procedure ensures that none Overwatch soldier is able to rebel against the Combine

Sites
Memory replacement could have been carried out in Citadel or in Nova Prospekt. Fortunatly, both the sites are destroyed by Resistance