When did they transition?

"An expatriate (often shortened to expat) is a person temporarily or permanently residing, as an immigrant, in a country other than that of their citizenship."

its a wordplay + bias by mainstream media.

expats = migrants from the 'elite' class. educated, wealthy, well-connected etc. most often referred to persons from so called '1st/2nd world' countries.

immigrants = migrants from the 'cattle' class. not-so-rich, varied education, mostly non-caucasians. most often referred to persons from so called '3rd world' countries.

Well, in his opinion it was not a specific point in time, rather a period during which their ancestry became white because of lack of D Vitamin

They are, by definition, immigrants.

What is an expatriate, someone who comes from a country you would visit on vacation?

According to the article, at least one of the laureates is now a US citizen.

I found this '/s' on the floor, I think it must be your.