The toughest part was in fact Japanese terminology. Truth to be told, I learnt this kind of things in English and had no idea for Japanese version of Identifier, Modifier etc.
Although I took extra care of those jargons, I strongly recommend new learners take priority on assimilating the nature of the words, not the words themselves.

Anyway, It’s nice that I’ve done it!
Actually I have an experience to contribute to a flash book a few years ago. For this translation, I feel great for the fact that I could make use of the both skill of Japanese I improved at the time and English I’ve been studying for years. As always, I believe the set of skills affects multiplicatively, not additively. Therefore, I’ll keep trying to acquire new skills and maintain those things though I should avoid confusion caused by too many things to handle.

Besides, through this translation I got a deep comprehension towards the basics of ActionSccript. Although it’s said that there is no royal road to learning, I know the one. Teaching it to somebody is by far the best way to learn the topic. Translating is in fact the process of digesting what others are saying and rendering it into Japanese. This definitely improves my skill as well. I’d like to have a next try of translation in the near feature.

nevertheless, it’s a sort of trade-off between performance and aesthetic. The more particles, the less speed or vice versa. Actually, it becomes 0.1fps or less than this when I use AE seriously and I’m challenging to achieve similar thing by real time flash movie.
Maybe I should a lot of trial and error to find out the best parameter for it and meanwhile optimise the algorism. Using AS3 might be faster than this but due to the drawing engine which is almost same as AS2, I can’t expect much. So, at first, I’ll do it by AS3 and after that, AS3 version might come.

When you are creating a timeline animation, you might want to swap symbols in a layer with keeping movement. Usually, what you do is select the item, press “swap” button and repeat these process. For me a few times are acceptable. However, around five times, I would get agitated, and around ten times I would feel as though it was a torture or something like that. Although it actually takes several minutes, I can’t stand this kind of tedious job psychologically.
Thus, with my motto “I’ll work hard to get efficiency”, I wrote an automation script.

How to use this,
Select a layer which contains target instance and select a symbol from the library.
Then, run the command. Unless an error occurs, it’ll be done.

You should consider these followings,
I intend to use this command for timeline animation. Therefore, if a keyframe has more than two object, an error occurs.
Obviously, you can swap only Movie Clip, Graphic, and Button.
This also doesn’t support bitmap and video.

Rights and agreements,
I won’t have any responsibilities for damages caused by this script, even if something should happen.
This script is licensed under the Creative Commons Attribution 2.5 License.

In fact I use this script for my ongoing project and it works pretty well. We should let computers do things which can be done by computers and spend our recourses for creative works. It reduces my stress too and I feel easy to have trial and error. I believe this kind of small thing contributes qualities and productivities.

9/23 added
the name has changed
to “swap symbols in the same layer”
from “swap symbols in a layer”