I was reading about Rest Energy and came across this line: "In special relativity, however, the energy of a body at rest is determined to be mc2. Thus, each body of rest mass m possesses mc2 of “rest energy,” which potentially is available for conversion to other forms of energy. The mass-energy relation, moreover, implies that, if energy is released from the body as a result of such a conversion, then the rest mass of the body will decrease." The bit in bold is what I am having trouble understanding. If we change the rest energy do we not change the rest mass? I am thinking of an electron. If that rest energy changes (converted to another form of energy) doesn't the rest mass change and therefore it is no longer an electron? (Which surely is not possible). Could someone explain what it means by potentially available for conversion to other forms of energy? (Or do I just need to read a better source?!) Thanks.