It gets kind of nuanced in defining what "from scratch" means.

I still have my brain and experiences, but (in these cases) I no longer have the shared context with people. They don't know how to evaluate me. So then I behave like myself in the new context, and over time they come to value that however much they do. But, during the times when I've actually done this, I've been surprised every time with just how long it takes; or flipped around: surprised at how valuable that common context and experience is, how much it's practically worth in any complex environment.

There are some environments where you can imagine this working much faster, e.g., if the only expression of who you are is with git checkins or something, you might quickly get a sense of how much a person is to be trusted. Most of human interaction is much less that way, though. I think people are often mistaken about the extent that this is true.