In the server GUI type in "/gamemode (playername) #" Don't put the () where it says playername, just put the player's name. Where the # is, you put either a 0 (survival) or a 1 (creative).
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/gamemode 1/gamemode 2 /gamemode 0
/gamemodesurvival/gamemode creative /gamemode adventure
(this only works if you have singleplayer commands or if your a mod or up in a multiplayer server) /gamemode (name here) (1 for creative 0 for survival) ex: /gamemode blabla34 1 creative or to switch from survival to creative and back just do: /gm
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.