I think it is when they got rid of their dictator Salazar in 74....

and thus became a democracy...

actually doesn't sound huge but it had a massive effect on their colonies in Africa.  an excellent book was written about the exodus from Angola.....

Another Day in Life  by Ryszard Kapuscinski 

Salazar's downfall it was the catalyst for 27 years civil war in Angola and I think around 16 years in Mozambique...so millions were effected.