Magnetic states in RVO3 (R = Y, Tb) were investigated using low-temperature and high-pressure neutron-diffraction techniques, and the pressure-temperature magnetic phase diagrams were clarified up to 6.2 GPa. We elucidated that, on application of pressure, the magnetic ground state changes from C-type spin ordering (C-SO) to G-type spin ordering (G-SO); this corresponds to the orbital ground-state switching from G-type orbital ordering (G-OO) to C-type orbital ordering (C-OO). It is also interesting that the G-SO transition occurs simultaneously with the C-OO transition, but not with the G-OO one. When the transition temperature of C-OO exceeds that of G-OO under high pressure, the G-OO phase vanishes completely, and a simultaneous spin and orbital order-disorder phase transition occurs. Such a transition is the first case in perovskite-type transition-metal oxides.