When a bond's rating is upgraded, the bond is considered a safer investment. This will lower the yield on the bond, because the risk premium is lower. The same principle is at work if you apply for loan. You can think of your credit score as your "bond rating". The better your credit score, the more of a safe bet you are to lend money to, and the more likely you are get a lower interest rate. The same is true of bonds. The yield of a bond is inversely related to the perceived risk of the bond not being repaid.
