I've just tried to survive in creative mode, but this did not work. What can I do?
If you have created a survival map and cheats are "off", you can't change the game mode
Is it possible to change that later?
If you are in the survival game then you have to go short menu> then start in LAN world there also activate the cheats
If you play in the 1.13 or higher you have to enter Survial or creative now. Unfortunately, numbers do not work anymore.
You can copy the world and change the settings. I do not know if buildings are still there. Can you try it? First copy the worlds folder as a backup to the desktop