I'm using the latest vim/gvim (8.0.118), but have noticed this problem in the past, with other versions. I created a file named fizzbuzz.java (outside of gvim), and opened it in a new tab in gvim on my Windows 7 PC at work, using the context menu of my file manager. After closing the tab, using :x, I discovered that I had to rename the file to FizzBuzz.java, to match the name of the class I wrote, which I did using outside of gvim. However, when I went to reopen it in the same instance of gvim, again using the context menu of my file manager, it reverts to the original casing, and when I close it with :x, it saves it as such, so that I had to rename the file again. It was only after deleting the buffer, and refreshing the buffer list in the menu that I was able to get gvim to correctly recognize the new casing of the filename.
This does not happen in gvim 7.4 on Linux (PCLinuxOS)
Is there an easier way to get gvim to recognize, and retain the change in casing of a filename on Windows?