Personally, I would love to have an inexpensive upgrade offer to a more recent device. But as I expect they won't be able to get the price under $100 to do that, I'm much better off if it's a firmware update.
I also have some really nice e-books from the MobileRead library that I'd like to continue to be able to read on the device, so I'd like to keep lrf support as well. But that's just a manner of inconvenience - I could also go download another format from MR to replace them.
What about a third option nobody has mentioned?... They might provide a book conversion utility. The 500 still reads lrf, but any new ePub book you buy can be run through the newly provided utility as part of the library software to convert it to an lrf, and then you can read it on your 500. This would clearly be a frustrating answer, but would seem to fall within the language we've seen as an "upgrade" path.