I currently have a version of berryboot dated Mar 14 2016 installed and running succesfully with 2 OSs.
I would like to update firmware. Is it safe to just replace that berryboot installation with the latest one (by writing files to the SD card from Windows) -- i.e. that will not lose my existing OS installs?
Yes this is fine.
I copied the new files over, including config.txt, after backing up the old files to my PC.
On reboot it prompted me to "install the OS" which was a bit scary; but I left it on the default option of "Use existing files". It completed this and rebooted; and I was successfully able to see and boot into the two pre-existing OSs I had before