For example, if I place a Yordle Snap Trap and then put a point into that ability, will the placed snap trap benefit from the higher level when it is activated?
There are a couple of champions who have "latent" trap-like abilities. Testing has indicated that most already placed traps benefit from leveling up, but others (specifically Maokai's saplings) do not benefit from leveling it up.
I coerced my friend FallenAngelEyes into helping me test this out with Maokai and his saplings (alas, I do not own Caitlyn).
This is from the level 1 sapling. Nasus takes 64 damage.
I then threw another sapling, then, after it was thrown, leveled up Throw Sapling to level 2.
This did 63 damage, so leveling up sapling toss after the sapling is already in the world has no effect. Compared to later testing, THIS IS THE EXCEPTION!
(Note:That sneaky Nasus gained a level between tests, and increased his magic resistance enough to mitigate 1 damage! Jerk.)
Finally, I threw a 3rd sapling, from level 2, to establish a baseline at the second rank. As expected, it did more damage.
But one ability on one champion doesn't tell the whole story. I then re-ran the tests with Teemo and his traps, and leveling the ability did update existing traps. Teemo and Maokai got different results!
Shaco, Caitlyn, and Nidalee's traps all scale retroactively as well.
The behavior seems intended that traps scale retroactivley - 'tis only Maokai (who was, unfortunately, the first one we tested) that doesn't adhere to this.