Replacing a shorter manuscript with a longer manuscript on arXiv I initially wrote a short manuscript (4 pages) intended for physics journal A and posted this short manuscript on arXiv. After referee feedback from journal A, I am now seeking to instead submit this manuscript to physics journal B but in a longer format (15 pages) with more comprehensive details and additional information/examples included. Given this significant update to the contents of the manuscript, is it all right to simply replace the original short manuscript on arXiv with the new longer manuscript? Or should I create a new entry on arXiv despite the central theme of the two manuscripts being similar?
On this point, if I submit the new longer manuscript to physics journal B, is it unethical if I don't update the original arXiv submission at all? (Namely, the short manuscript on arXiv and long manuscript published in physics journal B will have the same titles but contain differing contents.)
If the new manuscript version supersedes the old one, and includes all of its content, I would suggest replacing it. After all, if, say, PRL would have asked for Supplemental Material which would have added up to another 10 pages, you would also have updated the arxiv version.
An added advantage is that people don't get confused by the fact that there are two different versions of the same result on the arxiv, and they don't know which one to cite.  (You would also loose citations this way, except on google scholar, where you might gain some; this could of course be resolved by adding the Journal-Ref field for both of them.)
Finally, if you decide to post two different versions, it would be a good idea to add a corresponding comment field to both publications, informing the reader about their relation.