Based on this, it looks like Hearts do have a case. At the start of the season, rules state that the season will last 38 games, and clearly it did not.
Although the rules can be changed, I think there is a legal question on whether they can be changed when that change will have an immediate and detrimental impact on one specific member while giving an immediate and positive impact to another. At a minimum there is a case to argue on a duty of care to all members.