Warning: Minor SPOILERS ahead for The Umbrella Academy season 2.
Klaus reunites with his Vietnam War sweetheart Dave in The Umbrella Academy season 2, but the role has been recast with a new actor. Whereas Dave was played by Cody Ray Thompson in season 1, Young Dave is played by newcomer Calem MacDonald in season 2. Though there's certainly a resemblance between the two actors (and Klaus has no problem recognizing him), Dave was likely recast to account for the fact that Dave is still a teenager in 1963, when The Umbrella Academy season 2 is set.
To recap, Klaus was kidnapped by Commission agents Hazel and Cha-Cha in The Umbrella Academy season 1. He managed to escape and stole one of their time-traveling briefcases, opening it up and accidentally transporting himself to Vietnam in 1967. After materializing in a military tent, he was assumed to be just another soldier and forced into fatigues before he could explain himself. After that, Klaus spent almost a year fighting in the Vietnam War.
While stuck in the past, Klaus struck up a romance with Dave, a fellow soldier who took him under his wing. Klaus described Dave as the only person he'd ever truly loved other than himself, but their love came to a tragic end when Dave was killed on February 1, 1968, during a battle in the A Shau Valley. Klaus transported back to the present day immediately after Dave's death and was left traumatized and suffering from PTSD. The experience led to him going cold turkey and brute-forcing his way into sobriety (with a little help from Diego).
So, why was Dave recast for The Umbrella Academy season 2? Dave isn't actually that much younger when Klaus meets him again in 1963 - only four years younger than he was when Klaus met him the first time. However, he's still in his late teens and the original actor, Thompson, is in his thirties. It would have been a stretch to have him playing a teenage version of the character, particularly when Dave's youthful naivete is a core part of his character in The Umbrella Academy season 2.
Calem MacDonald, who plays Young Dave, previously appeared in the comedy-drama film Stage Mother, Canadian mockumentary series Trailer Park Boys, and true crime show The Detectives. Given how much the Hargreeves family loves to mess around with time travel, it wouldn't be surprising if Dave returned in The Umbrella Academy season 3 - whether played by Thompson, MacDonald, or someone else.