In 1971 the government declared an unlawful state of emergency that stayed in force for 6 years suspending civil liberties, suppressing press freedom and giving the executive wide powers. The constitution was updated, by the parliament, the same constitution prolonged the current government mandate for two years without elections.
What you're talking about are the events of 1978.
And, as you probably know, during the 80s, that presidential and authoritarian shift only got worse.
But my point stands, 1970s Sri Lanka, is still to date, the last parliamentary republic to turn authoritarian. Didn't cite this randomly.