The changes happened bit by bit, but no one noticed them. Or maybe no one tried to notice them. She noticed them in herself only because she was an inveterate keeper of records, written and internal. When she read her old diaries, she found the things she'd written alien and shallow. When she thought back over her life, there were lots of things she would have done differently had she been her current self then, rather than a succession of past selves. Opportunities she would have taken. Problems she could have avoided. Relationships she might have terminated earlier, before they turned sour.
Sometimes she felt proud of the changes in her self, sometimes she felt scared. Sometimes she felt like a fraud, like she was just pretending to be the person who did her job and knew her friends. She'd read somewhere that every cell in the body is replaced over eight years. That meant that some of the people she knew had, when they first met her, met a completely different person, physically. Not a single cell the same as now.
On a cold spring day she met an old friend from university outside a bookshop on the Charing Cross Road. She hadn't seen him for three years or so, and she didn't recognise him immediately. She'd passed him before he registered her and called her name. She turned when he called and after a couple of seconds his identity came to her and lit up a bright network of associations - parties, conversations, lectures, drunken nights out, misty winter mornings running late with an aching head. She smiled at him and who she used to be and said his name in greeting.
"You're as beautiful as ever," he said, and kissed her on the cheek.
She felt saddened that he could lie to her and not even know.