In the short term I suppose it means he continues onto the Holy Land with Edward and Edmund. I'm not sure if this would make much difference to the outcome of the Ninth Crusade, probably not. Though maybe he could be at Edward's side when the assassin comes for him and helps to fend him off without injury to Edward, that could possibly have an effect. Longer term it probably means a longer lasting House of Cornwall, a rather wealthy and powerful dynasty I would imagine. I'm not sure how this would affect Edward I's reign. Rival or loyal cousin? Depending on how long the male line lasts the Cornwalls could play a major role during the reign of Edward II.
On the other side of the equation there's the de Montforts. If the whole incident is simply avoided I wonder if they might eventually reach a rapprochement with Edward, maybe around the time their mother, Eleanor, dies in 1275. My understanding is that the murder of Henry totally dissipated any support they had in England while creating a kind of blood vendetta between them and Edward. Leicester's already been given to Edmund Crouchback so I don't know that the title could or would be restored to them but without the excommunication and bad reputation the murder earned them they could at least settle more successfully in southern Italy. So at least their line would continue, if not in England then in Naples.