Google Docs: Replace paragraphs' fonts with a new font

convertFonts.js

const IS_DRY_RUN = false

const FONTS_TO_REPLACE = ['Arial', 'Times New Roman']
const REPLACING_FONT = 'Helvetica Neue'

function convertFonts() {
  const doc = DocumentApp.getActiveDocument();
  const body = doc.getBody();
  const paragraphs = body.getParagraphs();

  // Define a custom paragraph style.
  const style = {};
  style[DocumentApp.Attribute.FONT_FAMILY] = REPLACING_FONT;

  let replacedParagraphs = 0

  for (var i = 0; i < paragraphs.length; i++) {
    const paragraph = paragraphs[i];
    const attributes = paragraph.getAttributes()

    if (FONTS_TO_REPLACE.includes(attributes[DocumentApp.Attribute.FONT_FAMILY])) {
      if (IS_DRY_RUN) {
        Logger.log('Found paragraph: ' + paragraph.getText())
      } else {
        paragraph.setAttributes(style)
      }

      replacedParagraphs++
    }
  }

  if (IS_DRY_RUN) {
    Logger.log(`${replacedParagraphs} paragraphs would be replaced`)
  } else {
    Logger.log(`Replaced ${replacedParagraphs} paragraphs!`)
  }
}