downgrade
To lower something to a worse or less important level.
To downgrade something means to reduce it to a lower level, rank, or quality. When a teacher downgrades your project score because you turned it in late, they're lowering the grade you would have received. When a storm gets downgraded from a hurricane to a tropical storm, meteorologists are saying it has weakened and become less dangerous.
The word often appears when something goes from better to worse. An airline might downgrade a passenger from first class to economy class. A company might downgrade its prediction about how much money it will make this year. A phone's battery health can downgrade over time as it gets older and holds less charge.
Downgrade works as both a verb and a noun. You can downgrade someone's access to a computer system (the action), or that change itself is called a downgrade (the thing). The opposite is upgrade, when something improves or moves to a higher level. If your family upgrades to a bigger house, that's moving up. If you later move to a smaller apartment, that's a downgrade.
People sometimes use the word more casually too. If your favorite restaurant changes its recipe and the food isn't as good, you might say they've downgraded their menu quality.