So I'm playing through Baldur's Gate 1, no mods for the first time in a decade. I created a half-elven lawful neutral ranger. Everything's going pretty well ditched Jaheira and Khalid (fighter made of paper) and picked up Imoen, Xan, Branwen, Danaheir and Minsc the team works well.

I just noticed hours and hours into the game that my alignment has switched from lawful neutral to neutral good. Which isn't really an issue, it's just surprising as I was under the impression that alignment switching based on choices didn't get introduced until BG2. Was this added in ToSC or some other update? I have to admit even though I did buy ToSC back in 99 I never really put much time into the extra content.

Rangers have to be a Good alignment, they can't be Lawful Neutral in the first place. If you're playing without mods, you wouldn't have been able to select LN as an option during character creation. Is it possible you were just mistaken, and he's been NG this whole time?

Possibly.