A few years ago, I remeasured our Forks XV 15 km course since the school parking lot had been completely reconfigured. The course started and finished on the school grounds and both the start and finish were routed through the parking lot. I changed the start and finish but did it in such a way that the mile and km marks were roughly in the same locations. When the race was over and the results sent out by our club e-mail person, she noted that this was a new record since the course was changed. Tom Carter, who had held the record since 1982, went through the roof. His point was that the course was basically the same. I agreed with him and asked the question to several people, some of whom are on this forum. The conclusion was that there were no hard and fast rules but it was a judgement question.
We decided to keep the old records.
Alan Jones