Uploading preprints of old Elsevier papers to arXiv, and updating arXiv preprints by newer versions
This is covered on the Elsevier website. Some key points:


  An accepted manuscript is the manuscript of an article that has been accepted for publication and which typically includes author-incorporated changes suggested during submission, peer review, and editor-author communications. They do not include other publisher value-added contributions such as copy-editing, formatting, technical enhancements and (if relevant) pagination.


and


  Authors can update their preprints on arXiv or RePEc with their accepted manuscript.


and


  Preprints should not be added to or enhanced in any way in order to appear more like, or to substitute for, the final versions of articles.