I sent mine for an upgrade and because of some unspecified problem received a refurbished 505, so I can't comment on the battery issue. I hate to advise you based on my preferences and experiences, but I'll throw a couple of thoughts out in case they'll help.
Since you are getting a 600, any books you buy from the Sony store (if that is where you get them) will be in EPUB format only; you will no longer be able to buy and download books from the Sony store that will open on your old reader without the upgrade. However, it is relatively easy to remove the DRM from the EPUB versions and convert them to LRF (Calibre software works wonderfully), which are readable on the 500. I was able to do this by searching some of the formatting forums on MobileRead.
For EPUB books that you buy elsewhere without DRM, you can directly convert them in Calibre.
You should be able to read your PDF files whether or not you upgrade.
In order to load books you bought in the past from Sony on your new 600, you'll need to redownload them from Sony as EPUBs (no extra cost). Fortunately, the Sony Reader software will keep both formats in your library, if you decide not to upgrade.
Finally, there is the possibility that Laredo won't be able to upgrade your device, and you'll end up with a replacement (may have to argue to get this). There are some features I miss about my 500, and I personally wish I had not sent it in for an upgrade.
My recommendation would be to keep your 500 as is for now and watch the calendar. If, after a couple of weeks with both devices (i.e., before the upgrade deadline runs out), you find that maintaining and converting two formats is unworkable, you can upgrade at that time.
I hope this is somewhat helpful!