In Special Relativity, changes in space-time, due to velocity, will generate changes in relativistic mass. Velocity does not alter rest mass. However, if rest mass is in motion with velocity V, the inertia and the momentum of any amount of rest mass, will appear to increase. This is why it would take infinity energy to move even a small about of rest mass at speed of light, even though the speed of light is finite. The mass seems to get heavier without the rest mass changing; relativistic mass.

Say hypothetically, we have had 1 kg of rest mass moving close enough to the speed of light that its relativistic mass has the equivalence of 1000 kg of mass; 1000 times momentum equivalence. If we put on the brakes, we end up with 1000 times as much energy equivalent as rest mass, given off as brake heat. With that much brake heat, the mass would glow, expand and even vaporize into sub-particles. The result is GR will now lower due to the mass density getting lower; expands. With the matter converting to free sub-particles, due to the heat, these will quickly convert to energy.

This scenario is able to convert rest mass to energy via added energy followed by a deceleration. The initial added energy for the acceleration toward C, to gain the relativistic mass, adds activation energy; see below. Velocity has the units of d/t, while acceleration has the units of d/t/t, the unit difference added was some extra time=t. What the scenario does is use time potential to convert mass M into energy. Energy has the units of d-t (wavelength-frequency). There is no mass left.

What keeps the sub particles lingering in the semi-permanent state we call rest mass, compared to the life expectancy of free sub-particles, is connected to time dilation stored with the sub-particles of composite matter. By adding additional time potential, above, I walked the sub particles time dilation, backwards, to place where they don't last as long; mass to energy.