The finding, by the Examinations Appeals Board, suggests that more than 7,000 pupils taking last year's A-level psychology exam were let down as the OCR board rushed to react to the previous year's regrading scandal.
OCR was wrong to change its marking rules at the last minute, when its own guidelines said that schools should study mark schemes before coursework was submitted, said the appeals board.
The board will now meet the Qualifications and Curriculum Authority to ensure the situation is not repeated.
However, the board refused to amend students' grades, saying that papers could not be re-marked according to the old mark scheme. The school which made the complaint said the verdict meant its year-long campaign was a "waste of time".
As The TES reported last October, King James's school, Knaresborough, was one of several to complain about the last-minute changes, arguing that many of its 53 students had lost a grade in coursework as a result.
The new mark scheme was more detailed and demanding, the school claimed.
Students could lose 15 marks out of 40 by keeping to the requirements of the old mark scheme, rather than the new.
Problems with OCR psychology coursework triggered 2002's exams regrading controversy, as schools complained that A-grade students had been graded U for the coursework modules.
OCR said that, as a result, it had been ordered to change last year's mark scheme, to increase the number of marks covering grades A to U. Schools had been notified that there would be changes in April 2003, but were not told what the changes were until July.
Giving its verdict, the appeals board said: "OCR had been under pressure to re-write the mark scheme as a result of the Tomlinson inquiry (into the regrading controversy).
"Nevertheless, it was unfair to the candidates to introduce the new scheme at that time. As far as possible, awarding bodies should not introduce significant changes without giving centres enough detail and time to enable them to adjust their preparation of candidates."
The appeals board also criticised OCR for failing to change teacher guidance on its website to reflect the new mark scheme. Bene't Steinberg, head of public affairs at OCR, described the mark scheme's late publication as "regrettable".
Oundle school, near Peterborough, also had its appeal upheld, but no results were changed. A third appeal against OCR is to be heard later this month.