According to Kevin Feige, they're using the same de-aging technology that they used previously. As summarized in Screen Rant:
In a new interview with Slash Film, Marvel Studios president Kevin
Feige talks about the de-aging of Michelle Pfeiffer through digital
technology in Ant-Man & The Wasp, and indicates the same technology
will be used on Samuel L. Jackson in Captain Marvel. "Well, I think
having the option is pretty amazing. And I think having the
technology and even without spoiling anything, Sam Jackson is shooting
a movie for us right now that takes, where he’s entirely 25 years
younger the whole movie," Feige notes, admitting that the fact that
actors like Jackson, 69, have aged well helps with the technology.
The full interview at Slash Film more clearly shows that "how much" has moved up to "for the entire movie":
Feige: Sam Jackson is shooting a movie for us right now that takes,
where he’s entirely 25 years younger the whole movie [Captain Marvel].
So that’ll certainly be the one–
SlashFilm: And Coulson, right?
Feige: And Coulson. So that’ll be the first one where it’s a
character for the whole movie, as opposed to a glimpse at a certain
period of time. It’s the whole movie.
I think Feige also addresses OP's concern from the comments:
Anyways, why would you assume they would apply the same techniques?
when he says, "And I think having the technology...." Usually technology like this is simple to conceive, expensive to design, expensive to implement, and cheap to use. Once you have it, the incremental cost of using it only goes down. Having improved their de-aging up to this point, there's no point in not using it when they need to.
And as Valorum points out in the comments, the people doing that work are getting paid whether they're busy or not; might as well keep them busy.