function myReplace(str, before, after) {

  if (before[0] == before[0].toUpperCase()) {
     after = after[0].toUpperCase() + after.slice(1, after.lenght);
  }
  alert(after[0]);
  str = str.replace(before, after);
  return str;
}

myReplace("A quick brown fox jumped over the lazy dog", "jumped", "leaped");