Reality is that, 14 years after the drafting of the Constitution of 1977, the Soviet Union ceased to exist.
The mechanism they used was secession.
From Wiki:
The increasing political unrest led the conservative establishment of the Soviet military and the Communist Party to attempt a coup d'état to oust Gorbachev and re-establish an authoritarian and strong central regime in August 1991.[11] Although foiled by popular resistance led by Boris Yeltsin[12], then the president of the Russian SFSR, the coup attempt led to heightened fears that the reforms would be reversed, and most of the constituent republics began declaring outright independence.[13] On December 8, 1991 the presidents of the Soviet republics of Russia, Ukraine and Belarus met secretly and signed the Belavezha Accords agreeing to dissolve the Soviet Union, and replace it with a loose, voluntary union, the Commonwealth of Independent States.[14]
Two weeks later, 11 of the remaining 12 republics signed the Alma-Ata Protocol formally establishing the CIS and declaring that the Soviet Union had ceased to exist.[15] Increasingly powerless in the face of events, Gorbachev resigned from his office on December 25, and the Soviet Union formally ended its existence the next day
http://en.wikipedia.org/wiki/Dissolution_of_the_Soviet_Union