[kriztw]

The old definitions are still essentially correct, so for teaching nothing really changes except the word "approximately" being inserted somewhere.

[hoten]

Right. So in other words, the SI unit definitions are for scientists wishing to affirm the base units, but the old definitions are superior for education purposes (well, except maybe for the prototype kilogram...)

[peter303]

The pre-prototype kilogram is essentially valid- mass of liter of water. But now in terms of physical constants.