Complex sentence: A Constitutional Amendment was accordingly passed which transferred most of the president's powers to the newly created post of prime minister and only then was Goulart allowed to take the oath of office as President, to serve as head of State only.
Split into simpler sentences: A Constitutional Amendment was accordingly passed which transferred most of the president's powers to the newly created post of prime minister. Only after this amendment was Goulart allowed to take the oath of office as President, to serve as head of State only.