If the House votes favorably on articles of impeachment, he is impeached. If the Senate convicts, he is removed from office. There is nothing in the Constitution that says that the high crimes and misdemeanors had to occur during office.
Of course it will never happen, but it is not impossible.
My comment was based on what Dershowitz has said - events prior aren’t impeachable (for Kav & Trump). Then again, impeachment is a political process and basically, whatever the Congress critters want to impeach on, they can. The Repubs will follow what Dersh said, the Dems will impeach on whatever they can make up.