A:  This uses the sBreakApart() function.

function ReplaceStr(BaseString, ReplaceThis, WithThis: string): string;
var
  t: TStringList;
  i: integer;
begin
  t := TStringList.create;
  sBreakApart(BaseString, ReplaceThis, t);
  if t.count > 1 then
  begin
    result := '';
    for i := 0 to t.count - 2 do
      result := result + t[i] + WithThis;
    result := result + t[i + 1];
  end
  else result := BaseString;
  t.free;
end;