Hi,
Someone lied to you ;)
Or maybe they just left out a detail or two. PIX 6.3 will run on a 520. The only requirement is that you have the 16 MB flash in the PIX (image is too big for the older 2 MB flash). Some older 520's only have the 2 MB flash card so you will need ot upgrade if this is what you currently have. If you are on a later 5.X version or 6.X already, then you have the 16 MB flash. We have required more that 2 MB flash in the PIX for quite sometime. Hope this helps.
Scott