The way it works is that almost all VSL libraries include a standard portion of the library and an extended portion of the library. With the full collections, the extended libraries add additional articulations, while in SE, the extended libraries add additional instruments. Buying a standard library first, and an extended library later requires two different purchases (note that VSL does offer a number of different bundles that combine related standard and extended libraries at an additional discount).
As for general discounts, owners of the various SE libraries do get discounts on the extended libraries of the full packages. Within VSL, in the end, one does not pay twice for the same samples. It is important to realize that the samples in the extended libraries are different from the samples in the standard libraries. What you are looking at having to do now is purchase the extended portion of your SE library. Honestly, the demo starts have, at times, caused people some confusion. Note that VSL recently changed their policy, and have removed the demo starts provision from new Vienna keys.