function CtoH(obj)
{
var str=obj.value;
var result="";
for (var i = 0; i < str.length; i++)
{
if (str.charCodeAt(i)==12288)
{
result+= String.fromCharCode(str.charCodeAt(i)-12256);
continue;
}
if (str.charCodeAt(i)>65280 && str.charCodeAt(i)<65375)
result+= String.fromCharCode(str.charCodeAt(i)-65248);
else result+= String.fromCharCode(str.charCodeAt(i));
}
obj.value=result;
}

<input  type="text" onkeyup="CtoH(this);">