Open chat, and then type "/gamemode #" (without quotes) Replace the # with:
0 (survival mode)
1 (creative mode)
2 (adventure mode)
(only works if you enabled cheats when you made the world)
Chat with our AI personalities
/gamemode 1/gamemode 2 /gamemode 0
/gamemodesurvival/gamemode creative /gamemode adventure
/gamemode a
You need the single player commands mod. Then you type /gamemode 1. This will change your survival world to creative. To put it back use /gamemode 0.
Yes, the day/night cycle doesn't change no matter what gamemode you're in.