I've ran into this issue a number of times when uploading new versions of files to the archives. While it appears that a co-author will be retained in new versions unless removed, this is not the case. Here's what you see when uploading a new version of the file:



I thought that a possible way of retaining a co-author could be to remove them in the UI, then add them again, however it seems that if they were an author on the most recent version of the file, there is no way to also make them an author of the new version being uploaded (without admin intervention, of course).

Not sure if this makes sense but I figured it warranted a bug report. Feel free to ask any other questions if I missed something.