Yes; I believe you must migrate your account to Mojang's website to to that.
Edit: Though, I'm not sure if it's possible without a premium account.
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A premium account is a Minecraft account that has bought the game. A premium account allows you to change your skin and play on multiplayer servers.
No you cannot change your minecraft email account, but it is possible to change your screen name by loging in to minecraft.net with a different computer...
Not that I know of. If someone gave an account away, someone else would change the password on it. You need to buy it.
If you mean Minecraft Classic, this is not possible. Minecraft classic is only meant for building, not survival. If you want to have survival, you'd have to buy the full version. To change day to night in Minecraft premium (full), as of 1.3 the command prompt was added to single-player so issue the command /time set night.
If the server does not have Online Mode enabled, users without premium accounts will be allowed to join. You can change the setting by editing the file server.properties with any plain text editor such as Notepad, then restarting the server.