Stuart Robert Kisby (18) and Darren Wheeler (19) left their victim with horrific injuries after repeatedly kicking and stamping on him.
The pair had taken drugs during an all-day drinking session prior to the incident.
Victim Stephen Swallow was walking through Lincoln city centre after a day spent playing golf and a meal with friends.
He was struck on the head from behind before the pair set on him and eventually left him lying on the ground soaked in his own blood.
Judges at the Criminal Appeal Court in London said they did not want to slash the men's youth custody but were legally bound to do so.
Mrs Justice Rafferty said that, because of the "explicit assurance" the Lincoln Crown Court judge had given the pair that they would be given credit for guilty pleas, their appeals had to be allowed.
She concluded: "That all three members of this court would have taken a very different view is sadly not the point. This court is constrained to remain loyal to the judge's indication."
After the hearing, Mr Swallow told the Echo: "My wife and I were left speechless. We feel it was a victory for criminal thugs"
Mr Swallow was in London yesterday to hear the Judges' ruling. He shook his head in disbelief when he heard that his attackers will be released earlier than originally planned.
After the hearing he told the Echo: "My wife and I were left speechless. We feel it was a victory for criminal thugs."
For the full story see Thursday's Lincolnshire Echo.