John McIntosh, who is 35 and from Aberdeen, was originally found guilty of attempting to murder a 37-year-old man in the Tillydrone area of the city in July last year.
However, appeal judges in Edinburgh agreed to reduce the charge to serious assault.
His sentence was reduced from nine years to six years.
During his original trial, the court had heard that McIntosh had lodged a self-defence case after claiming his victim Daniel Main had hit him on the head with a bat.
McIntosh claimed that he was unable to escape at the time because he was on crutches.
Mr Mane was later taken to Aberdeen Royal Infirmary and found to have sustained six or more blows from a hammer to his head resulting in a depressed skull fracture.
Following his conviction, lawyers acting for McIntosh raised a legal challenge, and leave to appeal was granted on the grounds that the trial judge, Lord Glennie, had misdirected the jury by failing to give guidance on the issue of provocation in the case.
His counsel, Jonathan Crowe, had argued before Lord Menzies - sitting with Lord Bracadale and Lord Turnbull - that the trial judge had been wrong to decide that the evidence could not support a plea of provocation.
Lord Turnbull said: "We accept the submission made on the appellant's behalf that the trial judge ought to have given directions on provocation and left open to the jury the opportunity of returning a reduced verdict on that basis."
Following their ruling the appeal judges quashed the conviction for attempted murder and substituted one for assault to severe injury, permanent disfigurement and impairment and to the danger of life.
In imposing a reduced prison sentence on McIntosh, Lord Menzies pointed out that he had a bad record, including numerous convictions for violence.
An appeal by McIntosh's former co-accused and then girlfriend, Joanne Mitchell who was jailed for two-and-a-half years for assault, was refused.
A man who was jailed for nine years for a hammer attack has had his prison sentence cut after a legal challenge.