I don’t think there is anything wrong with changing the schedule. Anything else would be quite dumb and probably just delusional. Perfectly predicting the future is not possible and if honest mistakes or simply unknowns lead to this then that’s just normal.

However, here “ahead of schedule” is used as a, well, I guess compliment and for that it’s important to know the context. That they adjusted the schedule is obviously not at all a problem and, as I said, for a project this big the couple years it took longer aren’t really that relevant even.