If you have a negative CON modifier, does every CON increase give health?

[Brutter]

If I'm playing a character with 9 Constitution, I'll lose 1 HP per level up, but if I roll a 1, my minimum health increase is still 1, so I don't lose that HP. If I then increase my Constitution by any method, I won't have a penalty any more, and I ought to retroactively get 1 more HP per level so far, right?

But if I rolled a 1 on one of my level ups, then I would only get 1 HP on that level anyway, even if I had 10 Constitution. Does that mean I'll retroactively get a +1 on that roll, effectively guaranteeing I get a minimum of 2 HP per level once I increase my Constitution?

And this gets even weirder with temporary effects. If I'm under the effects of Bear's Endurance when I level up, and I roll a 1, what happens? Do I start off with 2 HP for that level, until Bear's Endurance wears off, then I'll have 1 HP for that level, and then if Bear's Endurance is cast again I have 3 HP for that level instead?

[Khedrac]

If I'm playing a character with 9 Constitution, I'll lose 1 HP per level up, but if I roll a 1, my minimum health increase is still 1, so I don't lose that HP. If I then increase my Constitution by any method, I won't have a penalty any more, and I ought to retroactively get 1 more HP per level so far, right?
Correct.

But if I rolled a 1 on one of my level ups, then I would only get 1 HP on that level anyway, even if I had 10 Constitution. Does that mean I'll retroactively get a +1 on that roll, effectively guaranteeing I get a minimum of 2 HP per level once I increase my Constitution?
No - and this is why you should keep a record of how many hp you rolled at each level. Note this also applies to con losses as they cannot reduce below 1 per die.

And this gets even weirder with temporary effects. If I'm under the effects of Bear's Endurance when I level up, and I roll a 1, what happens? Do I start off with 2 HP for that level, until Bear's Endurance wears off, then I'll have 1 HP for that level, and then if Bear's Endurance is cast again I have 3 HP for that level instead?
As a general rule you ignore temporary effects when levelling up - they are not expected to last long enough, but if you do need to factor it in, roll the die, record the result and apply hit points according to current con - changing the total when con changes.