I believe the unprecedented part is no president has dissolved/eliminated a national monument and there may be no legal mechanism for one to do so - that will be up to the courts. Changing the boundaries of one is not the same, and when done for a public good, such as a right of way or a land exchange, has precedence.
And we can get silly here and change the boundaries to a 1 ft by 1 ft national monument, but again the presidents seem to be for the public good, not for reasons like i dam well feel like it and I'm the most important man in the world. The question seems to be did the founding fathers create a system good enough which will prevent being taken over by a ruler who believes he is one of the gods.