Later notable modifications included the Twenty-fifth Amendment, which was ratified in 1967. It explicitly stated (unlike the Constitution) that if the vice president assumes the Oval Office, he or she would be the president—not the acting president. It also set forth the process to determine if the president was incapable of discharging his or her duties. In addition, it provided for the succession of the vice president. Up until then, if the vice presidency was vacant, it remained empty until the next election. The Twenty-fifth Amendment, however, allowed the president to nominate a replacement, who would then need Senate confirmation. This provision was first invoked in 1973, when Gerald Ford replaced Spiro Agnew, who had resigned. And it was a good thing the vice presidency had been filled, because the following year Pres. Richard Nixon resigned.
In the ensuing years, some have pushed for additional changes. Critics have argued that the speaker of the House has a potential conflict of interest, since he or she plays a major role in any impeachment proceedings. Others have claimed that congressional officials do not meet the Constitution’s requirement that the president’s successor be an “Officer” of the United States. The document specifically states that officers are in the executive and judicial branches. In addition, some have noted that the speaker of the House and the president pro tempore could likely belong to a different party than the president, thus causing an unfair transfer of power. Despite such arguments, recent proposals to change the order have been resisted.
Sauce: https://www.britannica.com/story/the-surprisingly-disorderly-history-of-the-us-presidential-succession-order