Quoted: I was looking at some Lake City ammunition on Gunbroker, and the description says that it was demilled for civilian use by replacing the primer. Why do they need to change primers? Does this make the ammo any less reliable or accurate?

In 1997, Klinton signed a UN Treaty regarding the trafficing of small arms, and then signed an Executive Order to enforce that treaty in the US.  The EO ended the surplussing of all small-arms and related items (including mags and ammo) for civilian sales.

Since then, the only way for civilians to get surplus US military ammo is by having it "demilitarized", or "demil'd".  This means that a company who buys it must take it apart, and prove it by returning one of the components to the military.

Talon Ammo has a permit to purchase surplus ammo from the military for demil'ing.  They pull the bullets and powder, then remove the old primer and return it.  They then install new primers and reload the ammo.

The new ammo won't be properly sealed, and is generally loaded to lower pressures.  Sometimes it may have bullet setback problems as well, due to poor neck tension and/or lack of solid crimping.  Otherwise, the primers are new and the other components are unfired military surplus.

-Troy