There is some confusion in this thread regarding PRS-500's EPUB capability. Let me try to clarify some points.
When the firmware update (with EPUB support) for 505 was released, Sony said it's not "technically feasible" to do the same for 500 and instead offered the
trade-in program. Some people took it to mean that PRS-500 has not enough resources (such as RAM size) for Adobe's EPUB implementation. However, it was never clarified what exactly that meant.
I think it was more a decision of necessary resources (development, testing, documentation etc) versus possible return. At that point Sony eBook Store's switch to EPUB wasn't in the plans and PRS-500 owners could still get their books in LRF.
Now the situation changes. EPUB becomes the main format and if they don't provide some solution for PRS-500 owners, Sony faces the risk of alienating them. I think at this point it
can be "feasible" for Sony to produce firmware with EPUB support for PRS-500. I doubt it will mean removal of LRF viewer, which takes just around 600KB. Adobe's viewer is 6-7MB and necessary space can be easily found by reducing user space for books by just a bit.
Will they do it? Frankly, I doubt it. But it would be a nice gesture and I think Sony can afford it, even though it will probably not have any direct return.