According to another post [slashdot.org] this plutonium could not be used to make a bomb, and the explanation makes sense to me. So even if they change the constitution they won't be making any bombs, at least not with this plutonium.
This story rang some bells with me, and yes, it does appear to be the same case already reported on Slashdot (the figure given in the linked article there was also 640kg).
That time, however, the slant was on the Chinese being concerned that the Japanese may have been "stockpiling" this missing plutonium for weapons.
Which begs the question as to why, if it couldn't be used to make an atomic bomb?