I've been having a bit of a discussion involving the Thirsting Blade (Scaling Magic Items).
It starts with this:
This long, curved +1 dagger gives off a contented hum when soaked with blood. It can be thrown normally despite its length and has a critical threat range of 18–20.
Then it gives this:
8th Level: The thirsting blade is a +1 keen dagger.
The way I understand it, the 18-20 is an intrinsic part of the dagger itself, and not a magical modifier, meaning that making it keen would give it a critical threat range of 15-20. Some people have disagreed, saying that the critical threat range would be 17-20 (as a normal dagger would), and that this overwrites the 18-20.
I'd really like something official one way or the other, but I'm pretty sure that you aren't waiting 8 levels for a +1 keen dagger.
As a comparison: Armaggeddon Plate:
A suit of armageddon plate acts as +1 full plate for nearly any wearer—it automatically resizes when donned to fit wearers of size Small to Large. The armor is blackened, with joints and helm in a style reminiscent of the legendary tarrasque.
6th Level: Armageddon plate is +1 light fortification full plate.
8th Level: Armageddon plate retaliates against melee attackers when its fortification ability triggers. When an attacker strikes the wearer with a melee weapon and the armor negates a critical hit or sneak attack, the power of the armor leaves the attacker shaken for 1d4 rounds (Will DC 16 negates). This is a mind-affecting fear effect, and can stack with itself to make an attacker frightened or panicked.
HeroLab has this as a 15-20 crit range when you hit level 8. I would tend to agree with this as it is a special weapon with a normal crit range of 18-20, doubling that with Keen would be 15-20.
Yeah, that's a weird thing, but it seems to become 15-20 with keen.
