The bad news is that, in general, Secure e-books (with DRM) are not transferable between reader vendors. So Adobe e-books are only readable with Adobe software, and eReader e-books (say) with eReader software. This is a very sore point with many of us, because it does not seem right that an e-book can't necessarily be read on a new device. It does not help in your case, but the DRM on a few formats can be broken and so these formats are more future-proof than the others.
The good news is that Adobe Digital Editions (e.g. on a Windows PC) should be able to read your purchased e-books, and Adobe DE is supposed to be available on at least the Sony PRS-505 eventually. There is still no commitment from Sony to make this available on the PRS-500.
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