I saw that you can change you gamertag in game, like when the time cowgod switched to sLiMeY c0w. How do you do it?
Moderators can do /nick and give players a nickname. It's not really anything to worry about unless pinkfairymagic changed her nickname herself.
Changing your in-game name is currently not possible, unless you buy a new copy of Minecraft and pick a new name. Mojang said that name changes are "coming soon" but not currently possible. Note that when the name changes from Mojang do come, your name on Sandlot will not change. As for us moderators, yes we can give you nicknames which start with a ~.
How dare you! I was 110% you all were using magic! How could it not be magic? Now you've spoiled the secrets to all the little boys and girls. They now see through your liiiies. Shame. (Don't worry! I know they use magic and say funny words to make it happen! Don't believe what they say! They use magic! MAGIC) [Before I get yelled at, I meant this post to be funny, not in any way insulting or other. Completely funny and good-hearted.]