Q: What changes can I make to the manuscript after its acceptance?
I have recently received the manuscript accepted several months ago, and I want to add a relatively new quote. Is it ok? Can I add a new collaborator to become the author of the paper?
Requests for changes to an accepted manuscript are not uncommon. In fact, it is easier to make changes at this stage than after publication.
As in your case, such requests might be for changes in authorship (e.g., addition and/or removal of authors, changes to the order of authorship, and changes to the corresponding author of a manuscript) and minor changes to the text, labeling of display items, etc.
Changes in authorship, such as the addition of a new coauthor, is permitted as long as the new author fulfils all the criteria for authorship. If they have not contributed sufficiently to qualify being a coauthor, you may acknowledge them in the Acknowledgments section instead. Second, you might need to attach a signed declaration by all the current coauthors that they have no objection to the change, as well as a declaration from the added author that they approve of this version of the manuscript and take responsibility for the content of the manuscript.
As for changes to the text, they are permitted as long as they do not alter the basis for your conclusions. From your question, we cannot gauge how the “relatively new quote” affects your findings, interpretation, discussion, and conclusion. In your letter to the editor, please explain in detail what the new text is, why you need to add it, and how it does not affect the other parts of your paper.
You may see similar questions answered here:
https://www.editage.com/insights/can-i-make-a-correction-in-the-manuscript-after-it-is-accepted