Was Rian Johnson told the answers to The Force Awakens’ mysteries?

In advance of the release of his new murder mystery film Knives Out (which has been getting great reactions), the British Film Institute hosted a Screen Talk with Rian Johnson where he spoke about his career, including the making of his highly divisive entry in the Skywalker Saga’s Sequel Trilogy. According to a fan on Twitter, the director revealed that he in fact had many long discussions with J.J. Abrams about The Last Jedi.

So J.J. Abrams was actually involved with Episode VIII insofar as he talked with Rian Johnson about it prior to the film’s production. By the sound of it, these weren’t quick bits of small talk where the two directors just traded pleasantries as a matter of professional courtesy. Based on what Rian Johnson said, these were lengthy discussions about The Last Jedi.

We don't know what JJ Abrams and Rian Johnson discussed, but we do know that they did talk prior to The Last Jedi's production.

But the premise of the Q might be a bit faulty, because it implies that there is a retconn, when there may not be. You can't change something that isn't definitive, only add onto it and shape it.

Wikepedia: The Last Jedi

Johnson wrote the scene with the mirrored versions of Rey to symbolise her search for identity; when she asks for a vision of her parents, she sees only herself. Rey learns that her parents were "nobodies" as it would be "the hardest thing" she and the audience could hear; Johnson likened the scene to Luke learning that Darth Vader is his father in The Empire Strikes Back. He said: "The easiest thing for Rey and the audience to hear is, Oh yeah, you’re so-and-so's daughter. That would be wish fulfillment and instantly hand her a place in this story on a silver platter. The hardest thing for her is to hear she’s not going to get that easy answer ... You’re going to have to find the strength to stand on your own two feet and define yourself in this story.

But looking at The Last Jedi itself, it is only the word of a potentially untrustworthy villain who suggests to the audience that Rey's parents are "nobody" and where the Force Mirrors do not show us that this is the truth, as it chooses to only show Rey, Rey and two shadowy figures becoming one, suggesting that there is still a potential mystery behind the full truth of Rey's lineage. There was never any sufficient evidence to prove to the audience if Kylo Ren was right, only that he was playing on Rey's fears and loneliness. In other words, Johnson didn't want to give Rey and the Audience an "easy" answer, but it was never stated that this answer was THEE answer, just that it's a hard answer to hear...

However, Colin Trevarrow's Duel of Fates script gave Rey the surname of "Solona" and it's revealed that Kylo Ren and The Knights of Ren killed Rey's parents:

But despite its promise, Duel of the Fates doesn't exactly stick the landing in the end. Whereas Rise of Skywalker had a fairly tidy conclusion, this (alleged) version of the film ends in messy fragments. Rey is blinded by Kylo Ren, who has grown more powerful than any Sith before. She loses. It's revealed that it was Ren and his Knights who killed Rey's parents.

But there is no resolution as to why Rey having a surname is meant to mean anything to the audience, except that she learns the truth about what happened to them. But this might of played on Johnson's initial idea better for some.

IMO what happens in TROS better sets the stage for a spin-off where Rey can take the mantel of Luke's former role in "Legends" and begin a new Jedi Order, while also taking the baggage of The Skywalker Saga with her.

As to your comment about an interview regarding JJ Abrams mentioning that he has ideas for a bigger story, which some elements of The Force Awakens concept art made it's way into The Rise of Skywalker, it should be noted that from the get go, The Sequel Trilogy announced that each film would have it's own director and therefor JJ Abrams either only set a light tentative path and/or created his own head canon just to help him make The Force Awakens, as he was not intending to return to write another film in the trilogy.