I think this has already been said, more or less, although in a different way, perhaps. But to emphasise:
What OP suggests would be in no way a sex change, even were it medically possible.
-- This is because there is no one person in the story who could be thought of as undergoing the change. There is the person at the start, a woman (in OP's telling), who is then destroyed to be replaced by a man.
The woman does not survive the process -- she must be thought of as dying, as the man is brought to life.
Suppose a woman were to die at the same moment her grandson was born. No one would call this an example of a sex change, perfect or otherwise. A woman dies; a man is born. This is just what OP postulates.
So, no, Pop91 and other wannabe sex-changers: changing sex is not even theoretically possible. Sorry!
[Btw (and this might not be strictly on topic), the scenario OP describes does have echoes in recent philosophical literature, mostly following Derek Parfit in his book Reasons and Persons and elsewhere. Those of you who like thinking about such things might like to have a look. It is good fun, and interesting, at least to some of us.
You might notice Parfit and others consider personal identity, which, following John Locke I suppose, we can take to be about what makes a person the same person over time. -- Are you, or to what extent are you, the same person as that horrible child who peed on my uncle's carpet back in the day? (Or, perhaps more seriously, 'organised that transport of Jews to Auschwitz' so long ago?)
This connection between 'identity' and 'sameness' (they mean the same, roughly speaking) often seems to fall by the wayside in present-day discussion of identity in politics and gender theory to the detriment of clarity of thought in such domains, some would say. But enough for now.]