In heavy fermion materials, localized f electrons hybridize with itinerant electrons. Depending on whether the f electrons participate in the formation of the Fermi surface or not, the latter may be large or small. Whether the transition between a large and a small Fermi surface occurs gradually or suddenly remains an important unresolved question in heavy fermion metals. We report on a systematic study of the transition in the framework of the one-dimensional Kondo-Heisenberg chain, where an additional spin-exchange interaction is included between the localized spins. Using the density matrix renormalization group (DMRG) we demonstrate the existence of the transition, track its location, and analyze its nature. As couplings become large yet another transition is found into a phase-separated phase. The resulting phase diagram of the model will be presented, along with a variety of correlation functions between the conduction electrons and the localized spins.