Are you, are you?

There are days when I look in the mirror and
think, who are YOU, exactly, and
when did you get here?
And how?

This gradual change reminds me of the night we
saw Slowcoaster do the live switchover to
Jimmy Swift, so musicians were swapped out
One by One, subtly (well, seamlessly)

I have effected a seamless set switchover.

Now, whose life is this I/we find ourselves living?