What is the simplest way to change a Minecraft username for gameplay videos?

I know how to make a video, that's not the question. The question is, how would I easiest make a series of videos where I watch player A from the perspective of player B, and player A has a different username in each video? I would prefer to do this with player A and B connecting over LAN and without buying lots of different accounts, but I suspect it may not be possible. Do I need a modded server or can it be done without one?

EDIT: I have accepted an answer because it works to do exactly what I wanted, but frustratingly it also has a side-effect which means I can't use it - the visible skin of a player with a changed displayName reverts to default. I'd much appreciate a solution which lets me easily control both skin and display name easily, preferably under LAN connection.

For LAN games there's an easy way of doing this:

1. Locate your .minecraft folder (%appdata%/.minecraft)
2. Open the file launcher_profiles.json
3. Find the line "displayName": "Bob",
4. Change it to "displayName": "Carl", or whatever name you want to use
5. In the launcher it should now say Carl as well when starting the game

Restrictions:

• This does NOT work online. You'll get an error. You have to change back the name to the original one when you want to play online again.
• Inventory, Ender Chests, etc are tied to the name and a new name will clear them (until changed back).

CrushedPixel is your friend here:

He has made a Spigot plugin for this.

And a way to get it in Vanilla with command blocks.

Also, if you use the Replay Mod, you don't even need a second account to record from an external perspective.