Effects and stats changes to a previously activated ability
What happens if you level up, upgrade a skill or buy new items after casting a skill?
If you upgrade a skill while casting you’ll do more damage (if the ability has any), less cooldown to the next activation, and you’ll have paid only the energy required by the previous level of the ability!
The same applies to items. If you buy items you will receive the benefits of said items even for skills you already used that have not yet hit their target.
Let’s say you are using Gwen’s A/W skill.
At level 1, this ability has (as of patch 4.11)
Energy Cost: 50
Slow Strength 0%
At level 2
Energy Cost: 55
Slow Strength: 0%
By upgrading this ability at level 2 right after you activated it, you will use the upgraded stats (105 damage, 6.5s cooldown) but you’ll only have paid the 50 energy required for the ability at level 1.
Why? Because you pay the energy at the moment you cast the skill, but your damage is checked when you actually hit the target.
You can land a Celeste’s C to someone in the other side of the map, successively buy a shatterglass, and the damage of your already travelling C will increase due to the new item!
What about skills with special effects on overdrive?
It depends on how they work. By example, San Feng’s immunity to negative status on A and B overdrive starts from the moment the skill is casted. So if a Ringo’s C is coming to you, you trigger B and upgrade the skill during cast, you will not be immune to the burn effect.
However, Fortress’ B slow on overdrive is checked when you actually perform the 6 basic attacks required. So you can trigger the ability, perform 5 basic attacks, upgrade the ability to level 5, perform the last basic attack and you’ll get the slow.
Damage Over Time
This is also true for d.o.t. (damage over time), such as the burn from Adagio’s A. You’ll do more damage by upgrading the ability or buying items after have placed the burn on someone.