Mario saves Pauline in Donkey Kong / original game, and Pauline didn't come back into mainstream Mario games until Super Mario Odyssey (2017).
Princess Toadstool / Princess Peach seem to be the same character, much like Bowser / King Koopa, or Dr. Eggman vs Dr. Robotnik. The strange changing of names over the years has happened to other characters, but I think in-game cannon is that Princess Peach truly is Princess Toadstool.
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Still, with Princess Rosalina in Super Mario Galaxy, I'd say that Mario has definitely tried the "Princess Swap" move already.
Or really... Pauline was already swapped out for Princess Toadstool.