I received my upgraded PRS-500 from Sony this morning. Here's the first few observations:
1) None of the content was missing. All my books were still there.
2) So, apparently, LRF and LRX are still supported.
3) The menus on the reader (books by author, date...) now have alphabetical submenus (A-C, etc.)
4) I don't see any particular difference in speed or behavior except that when I call up a book or change font size, at least for the first time, there is a "formatting" process that takes place for several seconds.
5) There is still no support for the Mac. I tried installing the Library software for Mac, but the Reader is not recognized by the software. (I run the PC software in Parallels under Windows XP.)
I just installed Library 3.01 for the first time, so I haven't really tried out anything new other than putting an ePub file onto the Reader, which worked fine.
So, the very preliminary news that that nothing bad happens to the Reader with the upgrade. I'll report on anything more I discover if there is anything else to report.