  Changes of type transitions for the two-component hydrodynamic type systems
are discussed. It is shown that these systems generically assume the Jordan
form (with 2 X 2 Jordan block) on the transition line with hodograph equations
becoming parabolic. Conditions which allow or forbid the transition from
hyperbolic domain to elliptic one are discussed. Hamiltonian systems and their
special subclasses and equations, like dispersionless nonlinear Schroedinger,
dispersionless Boussinesq, one-dimensional isentropic gas dynamics equations
and nonlinear wave equations are studied. Numerical results concerning the
crossing of transition line for the dispersionless Boussinesq equation are
presented too.
