Soluția găsită de mine:

function myReplace(str, before, after) {
// convert before & after to array
if ( before.charAt(0) == before.charAt(0).toUpperCase() ) {
after = after.charAt(0).toUpperCase() + after.slice(1);
//after = after.split('');
//after[0] = after[0].toUpperCase();
//after = after.join('');
}
return str.replace(before, after);
}
console.log(myReplace("His name is Tom", "Tom", "john"));