Inflicting a trait if another trait exists.

[stagger] causes an enemy to lose 20% of his attack and magic attack, and is removed after an action.

But, if another [stagger] effect is applied onto the same enemy while the previous stagger exists, [stagger] will be removed, and changed into [Knockdown], where the enemy cannot do anything for the turn.

Take for example I have a skill called Stone. When it hits an enemy, it will inflict [stagger] for one turn.

Then I use a skill called Power Strike, that can also inflict [stagger]. The enemy's [stagger] is removed and changed into [Knockdown].

The same results should be achievable with 2 Stones, or 2 Power Strikes, or similar skills.

Is it possible to achieve this without scripts or plugins?

If not, what plugins do I need to use to do this?

Do I need to make a script?

you can use damage formula for that

check this thread

http://forums.rpgmakerweb.com/index.php?/topic/47099-rmmv-damage-formula-ideas-and-help/

if you still confuse... you can post your problem there and maybe someone have damage formula for you.

Make sure you use the correct terms. Traits and States are two different things - you're talking about States, not Traits.