A kilogram is defined by a metal cylinder in Paris
An old model or guideline was retired in favor of a new one.
TaughtThe kilogram is defined by the International Prototype Kilogram, a platinum-iridium cylinder kept in a vault outside Paris. It is the kilogram by definition.
NowOn 20 May 2019 the kilogram was redefined in terms of the Planck constant. The cylinder is now just an object. No SI base unit depends on a physical artifact any more.
What actually happened
Le Grand K had an embarrassing problem: it was drifting. Compared against its official copies over a century, it appeared to have lost roughly 50 micrograms. Since it was the definition, the honest way to state this was that the kilogram had changed mass and everything else in the universe had stayed the same, which is an absurd thing for a measurement system to say.
The 2019 redefinition fixed the Planck constant at exactly 6.62607015 × 10⁻³⁴ joule seconds and derived the kilogram from it, using an instrument called a Kibble balance that relates mechanical to electrical power. Any properly equipped lab can now realize a kilogram from first principles instead of flying a comparison to France. The same revision redefined the ampere, kelvin, and mole in terms of fixed constants. Your mass did not change. What changed is that the definition of mass no longer sits in a safe under three glass bell jars.
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Who was taught this
Still standard through 2019, so anyone who finished school between 1950 and 2019 learned the earlier version.