This does happen on occasion. It can be a source of tension between the Reptilians and the Anunnaki, but keep in mind that the human/Anunnaki hybrids are not viewed as equal to the Anunnaki themselves as a full-blooded member of that society. It is more that the hybrids are tolerated because they have some utility and need to be minded because they are aware of their extraterrestrial lineage, and that loyalty must be rewarded to some extent to keep them happy and willing to serve the Extraterrestrial Alliance. But that does not mean they are greatly valued as people think about human life, no matter who the individual is, having great importance and that it is something special. Because the Anunnaki are loveless, they cannot feel remorse or compassion for anyone, and in most cases when there is a replacement of such a hybrid it is because there is a need for greater control of that individual and it will actually serve the Extraterrestrial Alliance in all likelihood for there to be a Reptilian shapeshifter assuming their role. So a takeover by a Reptilian is more likely to happen when an Anunnaki human hybrid individual is becoming too soft, too sympathetic to what is being done to humans, and that cannot be tolerated.