Some details on BG2, yes, it did upgrade enemies sometimes.
It had several level character files. Partymembers had the most (so unlike modern RPG's where you can pick what you want, they join you at an appropriate level pre-defined). Enemies had quite a bit less.
In the end, since leveling takes long in Baldur's Gate II, and there aren't many levels (not 1-50 like most modern games), the system was barely actually used ingame. It did get in effect if you tried to important high-level PC's into the game from the start, but the thruth would be; the difficulty would still be horribly broken in the easy part.
It's not exactly the scaling most modern games do (since unlike systems adding numbers everything was preset and manually made). A LOT of work, you barely notice anything of it ingame, and as modmakers found out who made the unofficial patch, littered with hundreds and hundreds of bugs (it was quite obvious no-one ever tested the max-level stuff at the start of the game, and again; why would they?)
[End of History Lesson]
