The first thing is the Corleones would have found out almost immediately that Tatt was a front man for Barz who was consolidating his power to become the Don of all Dons. This would have been a direct threat to the Corleones. Further, Sol and Barz would have done exactly what the Rosatos did to Frankie...they would have started to move into Corleone territories and not only sell drugs, but get into gambling and maybe even the unions. Since Luca would have still been alive, he would have been dispatched to kill Sol, and this would have set off an assassination attempt on Vito, and from there the story would pretty much have been the same.