Relatives of soldiers killed while serving in acting roles will now receive their full pension for that rank, the Ministry of Defence has said.
From 3 January pensions will be paid on the basis of their acting rank, regardless of how long they had been serving in that position.
It comes after a campaign by the relatives of Sgt Matthew Telford from Grimsby who was killed in Afghanistan.
They had pressed for his pension to be reflected by his rank when he died.
The Ministry of Defence confirmed on Friday that Sgt Telford's family and those of two others servicemen would receive a £20,000 lump sum, reflective of a sergeant's pension.
Under previous legislation, death benefits were calculated on the "substantive rank held at the date of death", unless the individual had been in an acting rank for at least a year.
But a Ministry of Defence spokesman said as a result of Sgt Telford's case, death benefits from 3 January would be paid "on the basis of acting rank".
Sgt Matthew Telford, of Scartho, was one of five soldiers shot dead in Afghanistan by a rogue Afghan policeman.
Since his death, the widow, mother and other relatives of the father-of-two have campaigned for his pension rights to be based on his acting sergeant status, not that of a corporal.
The Ministry of Defence said the families were receiving the lump sum under the Armed Forces Compensation Scheme, as the new rules were not yet in place.
Sgt Telford's mother, Cheryl, welcomed the change.
She told BBC Look North: "I am obviously very pleased that the MoD have listened to our concerns and recognised that the 12-month rule is unjust.
"We felt that service personnel are killed in action, they should be recognised for the rank they held, and were proud to hold at the time of their death."
Mrs Telford said the family had received remarkable support from the public.
"It is quite refreshing to know that on this occasion people have listened and recognised the importance of what we were trying to achieve.
"It wasn't ever about the money, it was about the recognition of his rank.
"He was buried in his gold sergeant's uniform... There was some disparity there that I felt was unjust."