A lot of people won't trust information about the fraud from right-wing sources. But before the election, multiple left-wing sources like PBS made documentaries about how the voting machines were vulnerable to hacking and tampering. Politicians like Amy Klobuchar questioned the reliability of the machines. A politician in Georgia (I forget her name) was particularly vehement in her arguments against the machines.
But after the election, suddenly everything changed. Now the machines were 100% reliable, if the media and politicians were to be believed. PBS, Amy Klobuchar, the Georgia politician (who now famously said to Rudy Giuliani that evidence that had not even seen before was already "debunked"), etc, now suddenly viewed the idea that the machines might have been used to commit fraud as dangerous misinformation.
So if you want to redpill someone, start out by showing them the documentaries from left-wing sources like PBS about how the voting machines were easy to tamper with, videos and articles of left-wing politicians like Amy Klobuchar and the Georgia politician who complained that the machines were vulnerable, etc. Then, show them the things these same people said after the election for contrast, to demonstrate that they changed their position on a dime when it was no longer politically advantageous to them. From there on out, play it by ear, amping up the evidence according to what you think the particular person is now psychologically prepared for.
The comments section below would be a good place to compile any links you can dig up to left-wing documentaries, news articles, politicians, etc, who questioned the machines before the election and examples of what they said and did after the election. This compilation could then be spread around as a "redpill package" for people to use for their normie friends and family.
Some of the left-wing sources you speak of are featured in The Epoch Times' documentary on the 2020 election fraud, to do exactly as you have suggested, show their sudden change on narratives. They take it a step further and show how "experts" and professors also flip-flopped after preaching the election fraud narrative for a year or two.
Timestamps for that documentary are here:
https://greatawakening.win/p/12hR7bM5wK/the-epoch-times--election-fraud-/