Obi-Wan's claim that Vader killed Anakin Skywalker now only works from a certain point-of-view, while the Star Wars prequels explored their relationship much more fully - and the developments will continue in Obi-Wan Kenobi.
It's confirmed that not only will Darth Vader return in the show, but he and Obi-Wan will have a rematch, their third such duel in Star Wars canon after A New Hope and Star Wars: Episode III - Revenge of the Sith. The teases of this clash are exciting for how it'll go down in the show, but also intriguing for what it means for canon. There was no previous suggestion of a duel in this part of the timeline taking place, and though there's certainly enough wiggle room for it to happen without breaking things completely, it will at the very least re-contextualize events.
Arguably the main focus of concern on this has been the "learner...master" line from A New Hope, which would imply that Vader cannot win the Obi-Wan duel if that's to remain intact. That may be true, but even if he were to win it wouldn't necessarily change it - he begins the line with "when I left you," which can be taken as meaning when he turned to the dark side and away from Obi-Wan and the Jedi. There is, perhaps, a more interesting quote from A New Hope that Obi-Wan and Vader's rematch will change instead: "A tremor in the Force. The last time I felt it was in the presence of my old master." With Vader now definitely set to be in the presence of Obi-Wan in some way, then the line will take on a new meaning beyond what has previously been assumed: in A New Hope, that meant some distant past; after Revenge of the Sith, it seemed that was their final meeting he was referring to. Now, Star Wars can reframe it.
The Obi-Wan Kenobi trailer ended with a shot of the Jedi seemingly sensing the presence of Vader, smartly cut to come just after the latter's suit was being assembled and his iconic breathing was heard. Though it could simply be an editing trick, the intent was clearly to show Obi-Wan feeling Vader in the Force, perhaps a similar tremor to the one referred to by the Sith Lord in the first Star Wars movie. Obi-Wan Kenobi can take their connection further than ever before, developing not only their now broken friendship, but perhaps also the bond they have in the Force, sort of spiritually attuned with one another. It wasn't uncommon for Jedi and their Padawans to form strong Force connections, but Obi-Wan Kenobi could advance it more.
One of the biggest concepts introduced in the Star Wars sequels was Rey and Kylo Ren's ability to communicate (in a Force Skype way) and later even duel through the Force. Similarly, it's been established that dark side s can influence and bend Force bonds. They won't be a Force dyad like Rey and Kylo, but it's possible that Obi-Wan and Vader's rematch could include its own version of this kind of Force bond, with compelling theories that their fight will actually take place via the Force rather than face-to-face, which could certainly help avoid retcons further and give it the epic feel that's been promised.
With all of this, then the idea of Vader sensing a "tremor in the Force" that's similar to the last time he was in the presence of Obi-Wan can take on a much deeper meaning, and even properly show and explain it on screen and emphasize the point. Unlike the "learner," line, which has a "from a certain point-of-view" get out clause, the rematch in Obi-Wan Kenobi will almost certainly be the "last time" that Vader refers to in Star Wars: A New Hope, and it can change understanding of what that tremor in the Force was.
