Large and complex systems have a long-expected life and evolve slower than small systems. As such, they may live through several technological, social, economic and ecological changes in their environment. A fundamental challenge discussed in this paper is how to (re)design and change large-scale systems so that they remain maintainable and evolvable e.g. for the expected duration of their lives. This must also be achieved in view of their legacy and carried out in an affordable, risk-mitigated and timely manner. After defining some important features of large-scale systems and reviewing the state of the art in managing systems evolution, the paper characterizes the problems, solution scope and opportunities in the area and defines basic principles, theory, associated life cycle architecture and methodology approach for long-term systems supportability.