They can be rearranged without having to close them and reopen them in the order you want them. I know this because I work in 7 different workbooks every day, all day long. I have always opened them in the order I wanted, but I have noticed several times, that once in a while one of the workbooks that I opened has moved.
For example, just today, the 2nd workbook I opened is now the 7th one in the toolbar, so they can be rearranged, I just don't know what I'm doing that's making that happen, because when it happens, I'm not trying to do it, it just happens.
Once I figure it out, I will let you know.