Is it possible in Minecraft to switch over from survival to creative in single player?

As of Minecraft 1.3, singleplayer and multiplayer have been merged, so you can switch your gamemode with the /gamemode command:

• /gamemode survival
• /gamemode creative
• /gamemode adventure

You can abbreviate the modes. For example, typing /gamemode c for Creative works as well.

You can only do this if you haven't disabled cheats during world generation, though.

If you turned cheats off and still want to do it for whatever reason: The game mode is stored in the level.dat file, which can be edited using an NBT editor, such as NBTExplorer. The game mode is stored in Data/GameType.

I've found another tool (NBTEdit was unavailable when I tried to download it, so I poked around for alternatives).

The Game Mode Switcher Is exactly what it says on the tin.

It's a simple, external application with the added option of culling your inventory when you switch modes.

You can use /gamemode 0|1 now by starting your world with "Cheats Enabled".