Subject: Can the measured mass of an isolated system change

In a previous question (1093143254.Ph) I asked where the mass of potential
energy was, and the answer was that potential energy does not have mass.  This
raises another question:

Consider an isolated system where no matter nor energy can enter or leave. 
Inside this system, there are three objects: two permanent magnets that each
mass 1,000 Kg, and a block that masses 1 Kg.

Imagine that the initial state of this system is that the two magnets are stuck
together by their attraction for each other, and the block is just sitting off
to the side somewhere.  If you were to measure the mass of this system from the
outside, you would get a reading of 2,001 Kg.

Now imagine that you convert that block into energy.  1 Kg of mass equates to
about 9e16 J of energy.  You use this energy to separate the magnets from each
other such that there is 9e16 J of magnetic potential energy.  So in effect,
you've converted a 1 Kg block of matter into 9e16 J of potential energy.

Now, from outside the system, measure its mass again.  Do only read 2,000 Kg, or
do you still read 2,001 Kg?

It may seem that the answer to this can be deduced from the answer to my last
question, but that conclusion (that the measured mass of the isolated system
would indeed drop by 1 Kg) strikes me as being wrong.  I've always thought that
the detectable mass of any isolated system must be constant.  I'm submitting
this question to address that idea head-on.

Re: Can the measured mass of an isolated system change