Nuclear power stations is using the potential energy inside an unstable isotop. The energy will reduce the isotops mass, but remember that 1gram of mass is enough to power 1500 average housholds for one year (given that there is no loss). Litterally a grape that is converted to pure energy will power your house for 1500 years. Nuclear reactions is different from magnetism.
Same thing is applicable to field also. De-magnetisation only means dis-alignment of molecular magnets and not loss of magnetism. Magnetism still exists in a demagnetised magnet but magnetism of individual molecular magnets cancel off one another due to dis-alignment. When field is completely converted into energy it should produce same effect as complete conversion of mass into energy because mass and field are real and equivalent.
Whether mass has jumped out of field or matter produces field is not known. It is same as chicken & egg case.