I'm trying to figure out if and how alternate timelines could literally make Kyle Reese an entirely different person...
We know he hasn't been born yet as of 1995 in T2. Biehn Reese says he didn't see the war, he grew up in the ruins. Presumably he means he wasn't alive yet when the nukes were launched, since he was living in the war with the HKs and Terminators. T2 has the nukes starting in 1997 but this gets postponed when they destroy Cyberdyne and Dyson is killed. This means that Kyle Reese's parents would exist in entirely different circumstances, at least for awhile, than they did originally. And if they didn't even meet and conceive Kyle till after Judgment day then that potentially jeopardises the very existence of Biehn's Kyle Reese. And what if T2 set up new circumstances in which Biehn Reese's parents don't meet at all? A child could still be born of course but perhaps fathered by someone else entirely. Maybe the mum's surname is Reese and maybe she has always had first name 'Kyle' in mind - in that way a Kyle Reese still comes into existence but not Michael Biehn's one. Enter Jai Courtney.
That said, I don't for a second think this is the intention.