Before the Proclamation was published, there was no guarantee that a Union victory would change things in the South. After all, Lincoln did not go to war to end slavery in the South but rather to ensure that the Union would stay together. Lincoln famously told Horace Greeley that if he could save the Union without freeing any slaves, he would.
Thus, before the Emancipation Proclamation was issued, there was the possibility that the South could have returned to the Union without having to abolish slavery. This would not have constituted any sort of a social revolution. Once the Proclamation was issued, though, it was clear that a Union victory would mean the abolition of slavery and a major social revolution in the South.