#Java, 259 bytes

for (i=0;i<in.length();i++)
{
if(Character.isLowerCase(in.charAt(i))){
o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));}
else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));}
else {o=o+in.charAt(i);}}

Java, 259 bytes

for (i=0;i<in.length();i++)
{
if(Character.isLowerCase(in.charAt(i))){
o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));}
else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));}
else {o=o+in.charAt(i);}}

#Java, 244 bytes

for(i=0;i<in.length();i++){if(Character.isLowerCase(in.charAt(i))){o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));}else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));}else{o=o+in.charAt(i);}}

#Java, 259 bytes

for (i=0;i<in.length();i++) 
{ 
if(Character.isLowerCase(in.charAt(i))){ 
o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));} 
else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));} 
else {o=o+in.charAt(i);}}

#Java, 259 bytes

for (i=0;i<in.length();i++)
 {
 if(Character.isLowerCase(in.charAt(i))){
 o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));}
 else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));}
 else {o=o+in.charAt(i);}}

#Java, 244 bytes

for(i=0;i<in.length();i++){if(Character.isLowerCase(in.charAt(i))){o=o+in.charAt(i)+Character.toUpperCase(in.charAt(i));}else if(Character.isUpperCase(in.charAt(i))){o=o+in.charAt(i)+Character.toLowerCase(in.charAt(i));}else{o=o+in.charAt(i);}}