Fitch ratings agency has downgraded a series of Greek government bonds exchanged under a massive debt relief deal, but has left the country's overall rating unchanged. A Fitch statement says Greeces long-term rating remains at B- with a stable outlook. The agency said Thursday it downgraded 13 Greek foreign-law sovereign bonds, whose exchange was completed on April 11, from C to D. It said the ratings of the affected securities will be withdrawn on Friday, when the new deadline expires for a last batch of bondholders to participate in the bond swap. If carried out, that exchange will complete the biggest debt writedown in history, forgiving crisis-hit Greece just over half its €205 billion ($268.4 billion) debt held by banks, pension funds and other private investors. [Associated Press] -