An interesting question, excerpted as follows:
January 14, 2021 at 3:30 pm EST By Taegan Goddard 39 Comments
Roll Call: “The Constitution states, ‘When the President of the United States is tried the Chief Justice shall preside.’ Roberts presided over Trump’s first impeachment trial a year ago, just the third such trial in history.”
“But what happens when Trump leaves office on Jan. 20 — before the trial starts — and becomes a former president of the United States? Must the chief justice still preside over the trial? Could he choose to do so? Or does [that role] fall to the presiding officer of the Senate, the new vice president, Kamala Harris? Or can the Senate have its president pro tempore, at that time Vermont Democratic Sen. Patrick J. Leahy, preside? … Constitutional and congressional experts have no certain answer.”
https://politicalwire.com