When the concentration of a system is changed:

The equilibrium constant changes.

Either the numerator or the denominator of the mass action expression will change.

The mass action expression changes exponents.

The concentrations change to maintain the original value of K.

When the concentration of a system is changed, the equilibrium constant changes since the equilibrium constant is calculated using the concentrations of the reactants and products at equilibrium. Thus, when the concentration of the system increases of decreases, the value of K correspondingly changes.