I’m not sure they will admit they were wrong. People are more likely to deny they were ever supporters in the first place and destroy the proof.

One example is Charles Sherrill - he was an American aristocrat & politician in the 1930s. He had great respect for Hitler and was very much in favour of the US attending the Berlin olympics. Because he died in 1936, his journals remain behind as one of few examples of the rich class singing Hitler’s praises, and it’s clear that he wasn’t alone. However, anyone who lived long enough to see WWII and that their horse wasn’t going to win would have destroyed these documents and pretend they were just going along with things for social reasons even though they didn’t support them in their heart.