Program:

#include<iostream>
#include<fstream>
#include<cstring>

using namespace std;

// creating class for the operation

class Converter{
	private:
		// two string variables to hold file names
		char source_file[30];
		char targer_file[30];
	public:
		// constructor to initialize the file names
		Converter(const char *sf,const char *tf){
			strcpy(source_file, const_cast(sf));
			strcpy(targer_file, const_cast(tf));
		}
		void convert(){
			char ch;
			// open source file for reading
			fstream source_handle(source_file, ios::in);
			// open target file for writing
			fstream target_handle(targer_file, ios::out);

			// read the character till the end of file of
			// source is reached 
			while(1){
				source_handle.get(ch);
				if (source_handle.eof())
				{
					break;
				}
				// check if the read character is capital or small
				if (ch >= 65 && ch <=90 )
				{
					ch += 32;
					target_handle.put(ch);
				}else if(ch >= 97 && ch <=122 ){
					ch -= 32;
					target_handle.put(ch);
				}else{
					target_handle.put(ch);
				}
				
			}
			// close the file pointers
			source_handle.close();
			target_handle.close();
		}
};

int main(){
	Converter myconverter("source.txt", "target.txt");
	myconverter.convert();
	return 0;
}

Note: please create source.txt file with some contents in it then, run the program.