With either cold or hot fusion being made workable, need 10-20 years to put infrastructure up. Even so, oil producing states countries which have no other source of revenue (most of them) are in deep do-do as outside investment dries up because why invest billions in oil fields when you know that in 10-20 years or less, oil is going to be $10-20/barrel because demand for it as fuel (as opposed to chemical feedstock) is going to drop like a stone. "First world" countries are going to upgrade/computerize electric grids in anticipation to be ready when fusion plants come online, and you'll get a shift to electric cars/busses/trains as fast as manufacturers can put out usable models so that the major oil users will lead the way in dropping oil consumption. China & India will jump at this as both are oil importers. The "Third world" will come around more slowly, and they will be the last to go off petro fuel.
I'll leave the environmental benefits for somebody else. One knock-off is that the Arabs, especially the Saudis, will not have $$ to fund all the Wahhabi/Jihadist infrastructure from madrassas on up, and whakos like Chavez will be holding out tin cups rather than fuding other whaks.