The second change I believe was made during the first Bush administration and that was to include in the U.S. GDP those goods manufactured abroad so long as they were manufactured by American corporations or American-owned corporations. Suddenly the manufacturing GM was doing in Mexico and Ford was doing in Canada counted as U.S. GDP. The reason given for this change was the manufacture of goods that contributed to the bottom line of American corporations listed on American markets improve the health of American markets and thus the American economy. Critics pointed out this hid the amount of American manufacturing being moved overseas.
We can debate back and forth whether these changes are reasonable but one thing is clear - when comparing historical GDP numbers you're necessarily comparing apples to apples.
What's the old saying? Figures don't lie but liars figure? Like I said, the liars have been working on these figures literally for decades now. I think we've been covering up the damage brought about by Nixon's unilateral dismantling of the Bretton Woods Agreement.