Mind uploading speculation and debate often concludes that a procedure described as gradual in-place replacement preserves personal identity while a procedure described as destructive scan-and-copy produces some other identity in the target substrate such that personal identity is lost along with the biological brain. A paper tried to establishe metaphysical equivalence between these two methods in terms of preserving personal identity.
When does something stop being what is was ?
So from the perspective of the individual. Changes from most experience are not destroying their own persistent identity.
However, lobotimization is destroying personality and a significant loss (I would say critical loss of identity). The person might still believe it has continuity of identity but people who know the individual would say that personality and the person are different.
Mild stroke destroys part of the brain, but not enough that the brain cannot heal itself and replace the functions and personality and identity are believed by the individual and the people who know them to be preserved.
The Prestige movie (spoiler alert) had identical copies created of an individual. The instantaneous copies both exist at the same time, but then one kills the other. It is not known which version is killed. But from the perspective of the individual there is the one that existed before the copying process and then that one and another added. If I am the one copied then my identity and continuity is the essential property from my perspective. Having another copy is like having one part of a siamese twin exist after one dies.