The reason for the change is probably CPAC. Backing up a bit, the narrative was that with Trump's election loss, blame for also losing the Senate, and his subsequent bad behavior Trumpism was assumed to be dead. Put nicely by a congresswoman: "we Republicans need to stand for genuinely conservative principles, and steer away from the dangerous and anti-democratic Trump cult of personality" (Liz Cheney herself in a Washington Post May 5 opinion piece).
Going back to the second impeachment, in early February Mitch McConnell, the Senate Republican leader 'excoriated former President Trump [...] said the former president was "practically and morally responsible" for the attack on the Capitol [... and said] "This was an intensifying crescendo of conspiracy theories orchestrated by an outgoing president who seemed determined to either overturn the voters' decision or else torch our institutions on the way out"' (CBS news Feb 14th). Yow! Of course that's only the Senate, which was actually broken into on Jan 6th. The House Republicans weren't quoted as much since they weren't needed for that vote, but 10 of them voted to impeach Trump. That seemed like a pretty thorough rejection of "Trumpism". Liz Cheney's brand of 'back to conservatism' seemed to be in.
About her, she's not just some congresswoman. She's royalty -- the daughter of Dick Cheney, Bush's very influential VP and before that GHW Bush's Secretary of Defense during Desert Storm. She also got some conservative street cred coming out against gay marriage after her gay sister got married.
Meanwhile CPAC (the conservative political action conference) was announced as being in Florida (Trump's new home) on December 8th (Florida WKNG News-6). That's after calls to overturn the election results, but it's still almost a month before the Jan 6th events. The new anti-Trump sentiment could prevail. But when we get to the event it's a Trump-a-thon. From Feb 28th NBC news "The event’s multiday program was teeming with Trump allies and sounded, at times, like a Trump-themed airing of grievances, from false claims of an unsecure 2020 election[...]", and "he won CPAC's presidential straw poll". This was Feb 25-28, just a few weeks after Senate trial.
The swing away from early Feb rejection of Trump continues. From Liz Cheney's piece again: "On the floor of the House on Jan. 13, McCarthy said: “The president bears responsibility for Wednesday’s attack on Congress by mob rioters. He should have immediately denounced the mob when he saw what was unfolding.” Now, McCarthy has changed his story" and more recently (May 12th NBC news): "Multiple Republican members of Congress on Wednesday offered a false retelling of the devastating events that occurred during the Capitol riot, with one calling the entire event a “bold faced lie” that more closely resembled a “normal tourist visit” than a deadly attack". It mentions the much-mocked Rep. Clyde clip where he says "Watching the TV footage of those who entered the Capitol, and walk through Statuary Hall showed people in an orderly fashion staying between the stanchions and ropes taking videos and pictures, you know".
To sum up, in early February it looked like the Republican party was moving to the idea Trump was a 1-term president, who tried to overthrow the government, and was really more about himself than being a true conservative. It was time to move on and Liz Cheney looked like one of the new leaders for that. But the swing stopped and reverse. It took this long for it to play out. Cheney might have apologized for voting to convict Trump, instead she said she was serious about him being a dead-end. Post-CPAC, members might have calmed down and decided Trump is not the future. But instead both positions hardened until it came to a head last week.