Why do you want to upgrade? You need to understand that answer.
If you're going to spend some decent money, you should go and hear them side by side for yourself. Is the difference worth the difference in the money you'd get selling your 805S and buying the 805 Diamond?
In my own personal experience, I had the B&W Silver Signatures for years and years. In personal comparisions, I wouldn't have traded them for even the 802 Diamond.
I've always heard that the Nautilus and Silver Signature were the two best speakers B&W ever made (I'm only going up through the first Diamond series, I've lost touch since then).
If I were you, I'd go listen to the 805 Diamonds and decide for yourself if they're worth the cost of the upgrade.
But if I were you, knowing what I've experienced, I'd wait for a set of Silver Signatures to pop up on Audiogon and buy them. They're one of the classics in audio for a very good reason!
You may also be better off buying a strong and powerful amp to really push your 805S to see how good they are, you may be happier going that route.
Good luck in your search.
Chuck