on world change:

       if player's gamemode is not creative:
           set player's gamemode to creative
           wait 1 tick
           set player's gamemode to survival

       else if player's gamemode is not survival:
           set player's gamemode to creative
           wait 1 tick
           set player's gamemode to survival


#or

Every 2 seconds:
	if player's gamemode is not creative:
		set player's fly mode to true