How to replace text with a MS Word web add-in by preserving the formatting?

I would do something like this:

// get data as OOXML
Office.context.document.getSelectedDataAsync(Office.CoercionType.Ooxml, function (result) {
    if (result.status === "succeeded") {
        var selectionAsOOXML = result.value;
        var bodyContentAsOOXML = selectionAsOOXML.match(/<w:body.*?>(.*?)<\/w:body>/)[1];

        // perform manipulations to the body
        // it can be difficult to do to OOXML but with som regexps it should be possible
        bodyContentAsOOXML = bodyContentAsOOXML.replace(/error/g, 'rorre'); // reverse the word 'error'

        // insert the body back in to the OOXML
        selectionAsOOXML = selectionAsOOXML.replace(/(<w:body.*?>)(.*?)<\/w:body>/, '$1' + bodyContentAsOOXML + '<\/w:body>');

        // replace the selected text with the new OOXML
        Office.context.document.setSelectedDataAsync(selectionAsOOXML, { coercionType: Office.CoercionType.Ooxml }, function (asyncResult) {
            if (asyncResult.status === "failed") {
                console.log("Action failed with error: " + asyncResult.error.message);
            }
        });
    }
});