便利なのでメモ。
DEMO
index.html
<!DOCTYPE html>
<head>
<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<script src="jquery.js"></script>
</head>
<html lang="en">
<body>
<div>
<p><input type="text" class="js-characters-change"></p>
</div>
</body>
</html>
jquery.js
$(function(){
$(".js-characters-change").blur(function(){
charactersChange($(this));
});
charactersChange = function(ele){
var val = ele.val();
var han = val.replace(/[A-Za-z0-9]/g,function(s){return String.fromCharCode(s.charCodeAt(0)-0xFEE0)});
if(val.match(/[A-Za-z0-9]/g)){
$(ele).val(han);
}
}
});