var oldLink = "https://your-old-link-here";
var newLink = "https://your-new-link-here";
var documentId = 'YOUR-GOOGLE-DOCUMENT-ID-HERE';
var doc = DocumentApp.openById(documentId);
var searchType = DocumentApp.ElementType.TEXT;

function findAndReplaceLinks() {
  var body = doc.getBody();
  var text = body.getText();

  var searchResult = null;
  var searchResultTextElement = null;
  var searchResultText = "";
  while (searchResult = body.findElement(searchType, searchResult)) {
    searchResultTextElement = searchResult.getElement().asText();
    searchResultText = searchResultTextElement.getText();
    // Logger.log("TEXT: %s", searchResultText);

    var words = searchResultText.match(/\S+/g);
    if (words) {
      words.map(function (e) {
        // sanitize search terms for regex relevent symbols
        e = e.replaceAll("(", "\\(").replaceAll(")", "\\)").replaceAll("+", "\\+").replaceAll("*", "\\*");
        e = e.replaceAll("[", "\\[").replaceAll("]", "\\]").replaceAll("{", "\\{").replaceAll("}", "\\}");
        // Logger.log("WORD: %s", e);

        var partialElementUrl = null;
        if (e.trim() != "") {
          var partialElement = searchResultTextElement.findText(e);
          var partialElementText = partialElement.getElement().asText();
          var startOffset = partialElement.getStartOffset();
          var endOffsetInclusive = partialElement.getEndOffsetInclusive();
          partialElementUrl = searchResultTextElement.getLinkUrl(partialElement.getStartOffset());
          if (partialElementUrl != null) {
            var updatedUrl = partialElementUrl.replace(oldLink, newLink);
            if (partialElementUrl.includes(oldLink)) {
              Logger.log("REPLACING WORD: %s AT OFFSET: %s", partialElementText.getText().substring(startOffset, endOffsetInclusive + 1), startOffset);
              partialElementText.setLinkUrl(startOffset, endOffsetInclusive, updatedUrl);
            } else if (partialElementUrl.includes(newLink)) {
              Logger.log("ALREADY REPLACED WORD: %s", partialElementUrl);
            }
          }
        }
      });
    }
  }
}