Are SPARKLE, IRON, SILK and AMBER downwards-compatible with legacy Virtual Guitarists?

The quick answer is: Sorry - not really.

The new Virtual Guitarist is based on a completely new technology and content (it is essentially a brand-new product carrying the same brand) so there is no backward compatibility.

However, since the concept is not entirely different, you can for example replace legacy rock riffs from Virtual Guitarist, Virtual Guitarist "Electric Edition" or Virtual Guitarist 2 by re-opening the sessions and replacing your tracks by VG IRON. You'll just have to find matching styles and phrases. Your effort will be rewarded with much more realistic tracks and more sonic options for rock riffs than ever before.