The Two Convicts Will Be Eligible For Parole Earlier
Kevin Liverpool, 36, and his sidekick Junior Bradshaw, 33, had their jail terms reduced by three Court of Appeal judges.
Junior Bradshaw and Kevin Liverpool were arrested in June 2011 near the singer's house in southwest England. A sword, three knives, two hammers, masks and gloves were found in their car.
The pair had driven to Stone's home with a samurai sword, and plotted to rob and kill the singer, dump her body in a river and then flee the country.
At the trial it was revealed the pair crashed their car north of Bristol and then got lost as they hunted for Miss Stone's home -- even asking residents where she lived while showing them a picture of her they had in their possession.
Residents who spotted their damaged car being driven in a suspicious manner called police and the pair were stopped and arrested.
Judge David Bean said Liverpool posed a "high risk of serious harm to the public" leading to a "clear conclusion that a life sentence was justified".
"The purpose and effect of a sentence of life imprisonment is that if it is never considered safe for the offender to be released, he may remain detained for life, and that if he is ever released it can only be on licence and subject to supervision."
The judge said Bradshaw was "of exceptionally low intellectual capacity".
Stone, who burst onto the music scene in 2003 with her platinum-selling debut album "The Soul Sessions", was aged 25 and at home at the time of the planned attack in June 2011.
Given time spent in custody and the shorter jail terms, Liverpool could be free by 2017 and Bradshaw by 2016.