While discussing the possibilities and impossibilities of exchanging Royal offspring and heirs in times of turmoil is amusing, it doesn't bring you much further in search for the truth. What you accept as possible is your point of view and so called proofs usually are pretty flimsy. Looking at the usual suspects, though, might give you some new ideas as to whether it should or could have been done or not.
Princess Marie-Thérèse of France was born the only legitimate daughter of King Louis XVI and Queen Marie Antoinette. Together with her parents and her brother Louis XVII, she was imprisoned by the republicans in the Temple. Her illegitimate half-sister Ernestine wasn't imprisoned with them, though, and added another unsolvable puzzle to the French Revolution.
Louis XVII would have been King of France after the beheading of his father Louis XVI but for the fact that France was a republic at the time. He died while imprisoned in Paris; or maybe he didn't. The case is puzzling to this day, and conflicting evidence is offered from likely or unlikely sources.