Level up abilities while using them

What happens when I level up abilities (like Garen's W (Judgment), Morgana's W and E (Tormented Soil, Black Shield), etc...) while I am using them?

Does the damage increase instantly or the does the damage increase just in the next ability cast?

Example:

I am playing with Garen and use Garen's W (Judgment). While spinning, I level up and I increase Garen's W (Judgment) a level too. Does the effect of leveling up the ability happens while Garen stills spinning or the effect only will happen in the next Garen's W (Judgment)?

Solution 1:

All damage values in the game are calculated when the damage is supposed to take effect (i.e., not ahead of time, but "just in time"). This means if you level up a skill while it is currently being used, the remaining duration of the effect will have increased damage.

Another example would be the exhaust sumomner spell. Let's say champion X is using a ranged auto attack on another champion that would normally deal 100 damage. If X shoots the bullet and is then exhausted before the projectile reaches the target, the target will take 60 damage (40% less). On the other hand, if X shoots the projectile while exhausted, and the exhaust effect ends right before the projectile lands, the target will take the full 100 damage.

Source