can i rename a file without having to “Save As” all over again?
? - You only need to Save As once. So please explain your problem in detail (This only happens, if you don’t take care to open your renamend file next time you work with it). This question sounds that you misinterpret the recent document list (thumbnails on the right panel of LibreOffice Startcenter) as some kind of a file management opportunity.
And please: Add your operating system information, since in fact "Renaming a File* in general is a task to be done within the operating system and not with an application (File Explorer on Windows, Dolphin in KDE, Nautilus in GNOME, Finder in macOS… to name a few)
Depends on what you mean. The file manager of your system allows you to change the filename if the file is not open. When you rename like that, LibreOffice “recent documents” will still remember the old name.
“Save as” will refresh the list of recent documents, which is sometimes desirable, in other cases not so much.