I supposed they'd have to do something, since it would stretch credulity to have a 100-something year old Nick Fury look the way that Jackson does. I'd have gone with Fury receiving a watered-down version of the Super Soldier Serum, which extends his life.
1) (white) Nick Fury does have a version of the super soldier serum fueling him. It's called the Infinity Formula. It's not quite as potent as what's in Steve Rogers.
I'm not sure if the Sam Jackson version does or not.
2) The Marvel Universe in the comics is (was?) comprised of many parralel universes. Each has a #.
The one that most of all the stories/issues took place in is #616.
That's the home of white Nick Fury.
3) about 15 years ago or so Marvel launched a line of comics set in a different universe (I can never remember the # they gave it). They called the line "Ultimates".
It had an Xmen title, an Avengers title, a Spiderman title, etc. Familiar characters, just variations on the theme.
This is the home of the Sam L.Jackson Fury.
Fury looks alot like Sam L. Jackson in this universe because the artist was a SLJ fan.....
This turned out to be a really popular version of the Fury character.
So when they started making the Marvel movies, who better to get to play Fury on screen than the guy who's spitting image they were already using there in the Ultimates line of books?
4) Flash forward a few more years:
SLJ as Nick Fury in the movies is a popular choice. And one that ALOT of people have now seen....
So Marvel decides to put him into the main stream universe (#616) in the books.
(I'm sure there's also a dose of PC/diversity
involved, but it was mostly a $ thing. SLJ like Fury sells more & ties in closer to what someone coming to the comics from a movie watching background would expect.)
So they all but retire white Fury.
And then they introduce his previously unknown black son (who looks alot like a younger SLJ) who's also a super-spy type.
And here we are today.