If you alter someone's brain (by surgery, head injury, drugs, etc.) so that
The answer to this question depends on what one's criteria for personal identity are, as well as the nature of the changes in personality brought about by the envisioned brain manipulation. If, for example, one took personal identity to consist in psychological continuity, understood to consist in a continuation of interests, plans, projects, etc., then if one were to alter someone's brain and that person's personality were to change markedly enough that the person no longer shared the same interests, plans, projects with the person who entered the operating room, than the person would indeed have ceased to exist, as a result of the operation, and so one could be said to have effectively killed the old person in the operating room and caused a new person to be born. If, however, one takes identity to consist in bodily identity, then even in a case when a person's interests, plans, projects, etc., were to change markedly, provided that the person continued to occupy the same body, then the same person would persist. Now the deep question about personal identity, I think, is the extent to which one or the other or both criteria are relevant to identity. (Ought one to decide between the 'psychological criterion' of identity and the 'bodily criterion'? Is there a sense in which both criteria are relevant to identity?) I recommend anyone interested in the general topic of personal identity to read Marya Schechtman, The Constitution of Selves, a searching examination of literature on the topic of personal identity that elaborates a very plausible (and quite deep) position.