That's a completely different question, how you move characters into the future.
I think that future fic is plagued with problems, and moving your characters forward in time and having them grow and change is a landmine. I have one rule for my series: Nothing I do can contradict canon, and I keep them as absolutely in character as possible.
There are a thousand post-series scenarios that are consistent with canon and are in character. Some appeal to me, some don't, and I'm sure the same is true for you.
As to whether something continues to be in character, that's not a situational question, because they're not real people, they're characters. The question becomes, are they still recognizable as that character? Do they still have enough of the traits we know as "them" to come across as "them"?
The situations are just settings, they don't actually influence the characters because they're not PEOPLE, they're characters. The only influence that exists in the author, and it's the author's task to keep them in character while she or he builds the plot, themes, etc around the characters.
And I never said Brian and Justin aren't real! I think they are very real. I have a whole meta about the realness of imaginary characters, but I just spent a shitload of time trying to find it on my old LJ and I can't. I'll keep looking! Maybe I posted it in a community!