season 2 saw the Hargreeves children actually succeed in a mission, averting an apocalypse in 1963. Unfortunately, when they traveled back to the present day, they discovered the entire timeline had been changed.
The timeline seems particularly malleable in The Umbrella Academy, although less so in season 1 - presumably indicating it was maintained in part by the Commission. The Umbrella Academy's actions in 1963 have rewritten future history, and as a result Sir Reginald Hargreeves never assembled the mismatched family viewers know and love. Instead, he founded the Sparrow Academy, with what appears to be a completely different team of super-powered individuals. It makes sense he'd look for other candidates; Sir Reginald didn't seem especially enamored of the group when he dined with them. He had good reason, given Luther randomly tore his shirt open, while Klaus had what looked like some sort of spontaneous fit when he was possessed by Ben's ghost.
But the changes aren't restricted to the alone. Why did Sir Reginald Hargreeves abandon the idea of the Umbrella Academy? And why has he clearly taken a different, even colder approach with the next generation, who are still dressed in school uniforms - even though they're adults?
Diego Changed Sir Reginald's Personal Timeline
The problem is that the Umbrella Academy changed a lot more of Sir Reginald Hargreeves' personal timeline than would perhaps have been wise. They did so when they discovered the android "mother" who had cared for them throughout their childhood, Grace, was actually originally a real person. Grace was apparently a physicist who worked at NASA, and Sir Reginald met her due to his work with the Space Program. They became an item, with Grace accompanying Sir Reginald on social events, and it was clear Sir Reginald cared deeply for her.
Grace's story in the original timeline is something of a mystery, but it's reasonable to assume she and Sir Reginald stayed together for some time. Something happened to her - most likely she died, due to unknown circumstances - and a grief-stricken Sir Reginald replaced her with an android, called Mom. It's actually quite insulting that he replaced a fierce, intelligent, and independent woman with an android in her image he could control, and who serve as nothing more than a housekeeper and a mother figure to the children he had collected. It's yet another indication of Hargreeves' (lack of) character.
But, thanks to Diego's interference, Grace's history with Sir Reginald Hargreeves has been rewritten. He introduced seeds of doubt into their relationship when he told Grace her partner was attempting to assassinate President Kennedy during his visit to Dallas. Diego was wrong; while Hargreeves' co-conspirators in the Majestic 12 were indeed planning to kill Kennedy, Hargreeves wanted him kept alive. Unfortunately, a now suspicious Grace broke into Hargreeves' office and looked through his files, noting he had spent a great deal of time studying Kennedy's itinerary. Sir Reginald asked her to trust him, but Grace did not yet know him well enough to do so. No doubt the assassination of President Kennedy seemed to confirm her worst fears, and she cut all ties with Sir Reginald - always assuming she didn't attempt to tell the FBI about his activities.
What Would Sir Reginald Be Like Without Grace's Influence?
But all this raises a disturbing unanswered question about Umbrella Academy: what would Sir Reginald Hargreeves' life be like without Grace? The disturbing truth is that she probably helped to humanize him, assisting him in empathizing with the children he had gathered. It's reasonable to assume there's a reason Hargreeves decided to repurpose the body and personality of the woman he loved as a mother for the Umbrella Academy; because he sensed the absence of a mother figure in their lives, and decided some version of Grace would be the best for them. As distant and remote as Sir Reginald Hargreeves was in that original timeline, without Grace's influence he would most likely have been far worse.
That has profound implications for the Sparrow Academy. Even with Grace involved, Hargreeves' upbringing left the students of the Umbrella Academy badly scarred. They were hardly well-adjusted and severely lacking in social skills; as Klaus quipped in season 2, the best long-term relationship any of them had been able to commit to was when Five kept a shop window dummy with him in the apocalypse timeline. But without Grace, Hargreeves has probably been an even worse father-figure for the Sparrow Academy. This theory is ed by The Umbrella Academy season 2's final scenes, which showed the Sparrow Academy were adults still dressed in school uniforms - still viewed by Sir Reginald as children, intended to obey his every whim and serve his own mysterious agenda.
Grace appears to have been long-gone in the original timeline, replaced by that android doppelganger. That's not necessarily the case in the new timeline, however, because her fate could have been avoided simply by virtue of not being too close to Sir Reginald Hargreeves. As a result, the Umbrella Academy may well learn their mother is alive and well now, but in the flesh - and completely independent from Sir Reginald. Indeed, it's possible she even has Pogo with her, because - for all Sir Reginald referred to Pogo as "my monkey" in 1963 - he appears to have been particularly close to Grace. This would mean Hargreeves became even more extreme.
Diego thought he was trying to avert the JFK assassination while in 1963, but in so doing it looks as though he created a very different timeline - and no doubt The Umbrella Academy season 3 will enjoy exploring it. As such, it would be fascinating to see the Hargreeves children meet the real Grace, and get to know her as a person rather than just an android. Unfortunately, they may well find it impossible to restore the timeline once again - if they even want to, given that would potentially bring back the Apocalypse too.