I had the good fortune of having not one, but two flights cancelled BCN-JFK earlier this year. The flight was delayed, delayed, delayed then cancelled. And reappeared with one of those 4 digit flight numbers starting with a 9 and I learned it was so not to confuse it with the upcoming flights on that same route. It was still the same aircraft (I checked tail number). This happened for two days and they kept assigning a new number to the same aircraft. At some point I was like please rebook me on BA I will take the layover in LHR like I can’t do this anymore.
good luck with your travels ✌🏼