Comment basculer le cas des lettres en C?

Mon programme doit basculer les cas de lettres de certaines chaînes données. Par exemple, si les entrées sont les suivantes:

#include <stdio.h>

int my_len(char mystring[])
{
  int i, len = 0;
  for (i = 0; i <= 1001; i++)
  {
    if (mystring[i] != '\0')
    {
      len += 1;
    }
    else
    {
      break;
    }
  }
}

char upper_to_lower(char character)
{
    if (character >= 'A' && character <= 'Z')
    {
        return character - 'A' + 'a';
    }
    return character;
}

char lower_to_upper(char character)
{
    if (character >= 'a' && character <= 'z')
    {
        return character - 'a' + 'A';
    }
    return character;
}

int main()
{
    char list[10][1000];
    int i, count;
    scanf("%d\n", &count);
    for (i = 0; i < count; i++)
    {
        char temp[1000];
        gets(temp);
        strcpy(list[i], temp);
    }
    for (i = 0; i < count; i++)
    {
        //char f_temp[1000];
        int j;
        for (j = 0; j < my_len(list[i]); j++)
        {
            if (j == 0)
            {
                printf("%c", upper_to_lower(list[i][j]));
                //f_temp[j] = lower_to_upper(list[i][j]);
            }
            else if (list[i][j-1] != ' ')
            {
                printf("%c", upper_to_lower(list[i][j]));
                //f_temp[j] = upper_to_lower(list[i][j]);
            }
            else if (list[i][j-1] == ' ')
            {
                printf("%c", upper_to_lower(list[i][j]));
                //f_temp[j] = lower_to_upper(list[i][j]);
            }
        }
        //puts(f_temp);
        printf("\n");
    }
    return 0;
}