fiction - How best to handle revealing a main character's name midway through a long story in close-third person?

I'm writing a novel, and it's written in close third person perspective, very much sitting on the shoulder of one main character and privy to their thoughts. There is another very important character who goes only by his nickname for most of the novel, but who will mostly likely tell the main character his real name about a third of the way through the story.

My first instinct is to have the narrative switch from " Nickname said, 'why don't we do this?' " to " Realname said, 'why don't we do this?' " after the main character learns his name, but I worry that this will be hard for the reader to get used to after several hundred pages of Nickname and possibly even confusing. Nevertheless, once the main character knows the real name and has started to use it, it seems terribly awkward to keep referring to him by the nickname.

Is there any (relatively) canonical way of handling this / are there some good examples of either method?

Edit: Here is a set of 3 links to short articles on the close third perspective, just for reference on the difference between that and straight up 3rd person.

Answer

David Eddings does this in the Belgariad pentology.

Main character Garion is introduced to a man whom his Aunt Pol calls "Old Wolf," and Garion decides to call him "Mister Wolf." Mister Wolf later announces to other characters, "This is what Garion is calling me, and I happen to like it, so that's what you'll be calling me for now." All the attributives and narrative references to the man are "Mister Wolf." Garion later learns that the man is named Belgarath, and all attributives immediately switch over from there out. (This was so subtle I didn't notice it until my second or third read of the series.)

Similarly, Garion's aunt is "Aunt Pol" whenever he is the story's focus, but when the narrative turns to her when he's elsewhere, the attributives use her full name, "Polgara." This goes on until the end of the series, when Garion formally refers to her as "Lady Polgara" for the first time. After that, in the Malloreon (the sequel pentology), she is always "Polgara" no matter who is the narrative focus, because Garion has changed the way he thinks of her.

Interestingly, although at one point in the first series Garion does something to earn himself the honorific of "Bel-" and many characters refer to him as "Belgarion," the character himself always thinks of himself as "Garion."

There are also several characters who are spies, who tend to go by their nicknames: Kheldar is Silk, Liselle is Velvet. Khendon is Javelin. It's made clear early on what the characters' real names are, and they are used, but attributives are usually the nickname.

For your story, I would say it depends on how your main character thinks of this other person. If he thinks of the guy as "Bob" and later learns his real name is "Frank," it's fine to keep referring to him as Bob. But if he goes by "Nails," and Nails reveals his name is actually "Nicky" in a moment of bonding and friendship, then your character is going to think of him as Nicky.