The one man mainly responsible for the change in Rome's Government was Augustus. He first came to Rome after he was notified of Caesar's assassination. After confronting his enemies Mark Antony and Cleopatra in the battle of Activum, Antony and Cleopatra committed suicide and Augustus was left unchallenged by any man.
On his return to Rome he proclaimed he would restore the Republic, He reduced the number of senators from 1000 to 800 and he to the title of Princeps Senatus, which gave him authority to reshape the government. .
In 27BC Augustus was granted his seventh consulship and consented to accept the commission from the Senate to administer as a proconsular capacity. And they"re within lies the transition from Republic to Empire. Augustus also made other numerous changes to the government that would be successful for the next 500 years.
Augustus established a form of government known as a principate, which combined elements from the republic with the traditional powers of the monarchy. .
According to Edward T. Salmon, the Principate was intended to be a diarchy; a joint rule of Emperor and Senate.The Senate still functioned, though Augustus remained in control of the government. Most of the offices from the republic remained, although their functions changed. The most significant change to occur was that imperium (absolute power) was now reserved for the emperor. Advancement in rank was now based on conditions laid down by the emperor, rather than competitive campaigning. Therefore the cursus honorum changed from a ladder of power to one of social rank and status. The cursus honorum was only reserved for men with women forbidden from holding any sort of political office. Women obtained a kind of indirect rank through their roles as mothers, wives and daughters. Elite women sometimes claimed the rank of their fathers or husbands.
Source A: The Government of the Roman Empire under early Principate.