The function “getTagFromIdentifierAndTitle” is used by SharePoint particularly for changing form fields. But it is good enough only if you have straight forward fields. If you have enabled “Fill-in” values, it will not be able to find the correct element. Here is a extended version of this function to get the “Fill-in” enabled field.
function getTagFromIdentifierAndTitle(tagName, identifier, title) { var len = identifier.length, colonindex, splittitle, taglen, titlelen = title.length, slen; var tags = document.getElementsByTagName(tagName); for (var i=0; i < tags.length; i++) { var tempString = tags[i].id; taglen = tags[i].title.length; if(taglen<titlelen) continue; splittitle = tags[i].title.replace(title,""); slen = splittitle.length; if(slen == taglen || (slen > 0 && splittitle.indexOf(":") == -1)) continue; if ((identifier == "" || tempString.indexOf(identifier) == tempString.length - len)) { return tags[i]; } } return null; }