Unfortunately Portuguese communist government would of been besieged by security issues on the mainland and had its hand tied to launch any attack. Plus part of military that was opposed to communist could of moved to islands.
In 1974 when dictatorship wasoverthrown in revolt the Portuguese government looked like it was going taken over by communist. If that happened both Azores and Madeira would of declared independence and sought help from Britain and US. Portuguese communist government would at time be rocked by revolt from both centrist and right wing elements of military and population. It would not of been in any position to try and launch military invasion of either island group. By time communist are either in full control or had been defeated the Azores and Madeira would of been recognized by all of NATO countries and most Latin American countries as independent.
None of this is written in stone. Things could go in many ways.
I don't think revolts in the mainland are likely. The vast majority of the population at the time subscribed to some sort of leftist ideology, being it communism or socialism in some other form. Practically everyone who lived south of river tagus would support the new government for sure, and any rightist insurgencies that could happen in the north would be rapidly crushed since the right was very divided and deorganized.
Also, I don't think Britain and the US would be that quick to commit such an agressive act against Portugal as recognizing the independence of two of its rightful territories and sending troops there.
I think they're far more likely to try the diplomatic road first. As Loli American said, the'd put on an embargoe and try to peacefully talk the Portuguese out of communist.