You can do this on servers by quite simply typing in /gamemode 0. /gamemode 0 sets you into survival mode, /gamemode 1 sets you into creative mode and /gamemode 2 sets you into adventure mode. In singleplayer, if you didn't enable cheats when creating the world, you can press Esc and then click on the 'open to LAN' button. When you do this you will need to turn cheats on and then click on 'Start LAN World'. Now you can enter all the commands I previously said.
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Build the map on Creative Mode then go to seedscope.net and upload your level.dat for the map on the Change Survival to Creative option. You should then download the new level.dat. Replace it with the old level.dat and when you start Minecraft, you should see that it is now a survival map.
The 3 existing modes on minecraft are adventure, survival and creative.
you cant
You can't wear armour in creative mode. All you can do is infinitley place and destroy blocks.Actually you could by hacking minecraft... There might be a mod that can help do it that you could download, or you could just go into survival and put it on and then go back into creative.
/gamemode 1 yournameHit the chat key and type it in. Use 0 to go back to survival.