kirill_vasilevich,
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<script>
document.addEventListener('DOMContentLoaded',function(){
document.querySelector('input').addEventListener('input', function () {
document.querySelector('div').innerHTML = this.value = this.value.replace(/a/gi,"A");
})
})
</script>
</head>
<body>
<input type="text">
<div></div>
</body>
</html>