The court originally issued death sentences to the men for murder and other offenses in the February 2014 attack on the anti-government rally, but reduced the penalty because they confessed.
Relatives alleged the men were tortured into confessing after being taken into military custody in July last year, following an army coup in May that ousted an elected civilian government.
The attack and others on rallies held by the People's Democratic Reform Committee, which sought to topple the government of Prime Minister Yingluck Shinawatra, were cited by the military as a reason for their takeover.