When does upgrading you PC become a "new" PC?

I was wondering when people consider themselves to have a new PC. Technically I’ve had the same PC for close to 20 years now, but every part’s been upgraded several times over. I figure everyone’s got a different mind about it. For me, I’d have to say when all of the big three—CPU, GPU, mobo—have completed a phase, my brain thinks of the previous setup as “the old PC”.