Not every one:

As you play, you’ll leap forward with all kinds of named NPCs, right? Many of them won’t amount to anything, they’ll be just names, quick characterizations, simple motivations. That’s fine.
Whenever an NPC develops agency, though, list her as a threat

In my game some NPC isn't a threat (yet?), so I was also wondering if this makes any difference.

Hi all.

When I make another character as improvement, it has to be a brand new character, or can I also "upgrade" any NPC to PC?

If this were any other game, wouldn't you just make an on-the-spot ruling?

I'd search the rules for an answer, then - if I found nothing - look for a temporary solution with the players; afther session ending, I'd search the Internet and/or ask to the author.

Edit: With "look for a temporary solution with the players" I meant I'd make up a solution, checking if the players are OK with that; I didn't mean infinite talking. But the part "searching the Internet and/or asking to the author" is the important one and is why I'm here.

Do what? Chough when a round object of metal is forced inside his throat?

I didn't understand by "couching" you meant "coughing"; it doesn't change the meaning of what I (and Aetius) am saying: stab Doule in the eye, shot him point blank with a 3-harm gun, put a grenade beside his head... the point is: if that happens, it's deadly.

If Chaplain goes aggro and misses his move, the MC can make a move as hard as he likes

Yeas, but that means that Chaplain has to miss; what if he didn't? The grenade is in Doule's mouth and it explodes.
And, as I said, Doule is unconscious, so no roll is required (again: page 165. Bran is helpless, Keeler doesn't roll). No roll, no miss, no hard move.

Discussing about this on an Italian forum, I explained my doubt (that is Aetius') with an example: Chaplain is standing beside Doule, who is unconscious, and puts a grenade in his mouth. BOOM!

By the rule, this is 4-harm with the chance to take a disability to avoid any past-9:00 harm.
By the fiction... how can Doule survive? His whole head blew up.

If I look to the fiction, the second option (in Aetius' list) seems to me the only reasonable, if the player can't say how taking a disability can save his unconscious character from a granade exploding in his mouth; but obviously this ignore harm and clockdown.
Than I noticed this (page 178): "The players’ character sheets, like your front countdowns, are both prescriptive and descriptive. Prescriptive: changes to the character’s sheet mean changes to the character’s fictional circumstances and capabilities; that’s the game’s experience and improvement rules, following. Descriptive too: when the character’s fictional circumstances or capabilities change naturally, within the character’s fictional world, the player can and should change her character sheet to match".
So, I was wondering: "Your head blew up" can't be a change in the fictional circumstances, bringing to a change (to 12:00, if not dead outright) in the character sheet that doesn't follow harm rules? It seems to me the situation'd be quite descriptive of that change.

Edit: Doule and Chaplain are both PCs (a faceless and a gunlugger).