2002 Constitutional Amendment.
The 2002 Cuban constitutional amendment changed 3 articles of the constitution:
It was decided that the following paragraph should be added to the current Article 3:
"Socialism and the revolutionary political and social system established in the Constitution and proven through years of heroic resistance to aggression of all kinds and economic warfare waged by the successive administrations of the most powerful country that has ever existed, and having demonstrated their capacity to transform the country and create an entirely new and just society, are irrevocable; and Cuba will never again return to capitalism."
Prohibiting international negotiations while Cuba is subject to “aggression, threat, or coercion from a foreign power,”
Adding a reference to the 2002 changes and repeating the 2 basic statements above.
See: http://pdba.georgetown.edu/Constitutions/Cuba/cuba2002.html for a full version in Spanish.