I was *the* expert at my company for our original products -- standalone database applications developed in the days of MSDOS. Although I was second-generation (apart from a smaller application I wrote myself), the original developers had long since moved on and for 15 years I held court. I knew every source file and every bug by name; I could crank out modifications at a moment's notice; I was sole technical support for the final 5 years of the products' existence.
Suddenly management realized that we didn't have enough customers on the legacy products to need me doing a combination code maintenance / support any more, and there was a scramble to find a new spot for me.
Look upon me with horror and beware, kids: Don't wait for your job description to change before you start learning new technology. Dive into it NOW, so you'll be ready when your current skill set is obsolete.