Here's why Luke Skywalker changed his appearance and lightsaber to confront Kylo Ren in Star Wars: The Last Jedi. When Rey went to Ahch-To to find Luke, she was surprised the Jedi Master had adapted a jaded and disillusioned outlook on life. Due to his failure with Ben Solo, Luke firmly believed the galaxy would be a better place without him or the Jedi, so he willingly chose to spend the rest of his days in exile on the remote planet. However, his experience with Rey and a pep talk from Yoda encouraged Luke to aid his friends one final time.
The Last Jedi's third act is punctuated by a sequence where Luke arrives on Crait at the Resistance's moment of need. He goes out to face his nephew, allowing General Leia and the others to escape the First Order's clutches. There was an intriguing twist. Luke wasn't physically on Crait; he instead had used the Force to project himself across the galaxy, exerting whatever energy he had left in a self-sacrificial gesture. The Luke projection has a younger appearance and wields his classic blue lightsaber, and there are a couple reasons why that is.
The choices to make Luke look younger and give him a different lightsaber benefits the audience. It clues them in that not all is what it seems, which in turn makes the eventual reveal seem fairer. Initially, it's easy for viewers to get caught up in the moment of Luke's triumphant return to the fold, but especially on rewatches, director Rian Johnson's intent becomes clear. Luke's entire sequence on Crait contains several subtle hints that he's just a projection (including the fact he leaves no footprints on the planet surface). His appearance and lightsaber color are just part of the plan, so that when audiences find out Luke is physically still on Ahch-To, it feels less like a bait-and-switch and serves more as confirmation of what people suspected.
There is an in-universe rationale for this as well. Luke purposely made these design choices because he knew he was going to fight Kylo Ren. Luke wanted to goad his nephew by stirring up Kylo's feelings of hatred and rage. Luke's projection not only resembles the version Kylo knew (and remembers), but also Kylo has a very strong and personal attachment to the Skywalker lightsaber. As seen in The Force Awakens, he firmly believes he is the weapon's rightful owner. Based on Kylo's reaction to seeing Luke, it's safe to say Skywalker accomplished his mission. Kylo was distracted by his uncle and let that get in the way of his task. As a result, what was left of the Resistance was able to get away and lived to fight another day.
If Luke had decided to make his projection appear older, as he looks for most of The Last Jedi, it probably would have had a similar impact on Kylo Ren. Early on in the sequel trilogy, Kylo was obsessed with finding his uncle and ending the Jedi, so it really wouldn't have mattered if Luke looked younger or older. But the projection's design complements Luke's objective. It isn't coincidence the projection calls to mind the "younger" Luke that had briefly contemplated killing Ben Solo in his sleep. Conjuring up that painful memory set Kylo Ren off the deep end, thwarting his attempt to crush the Resistance.
