SWF optimization - heavy graphics

I’m coding games that come with the graphics done. Unfortunately the designer has some bad practices, such as not using Sprite symbols - never.

And the files have dozens (even hundreds) of symbols in the library, very few of them with any animation.

I would like to re-brand such symbols as Sprite to save some CPU (and maybe even kB), but my graphics-related skills are very low.

Where should I start? Check and convert the symbols at authoring time or it’s possible to do this at runtime? Can I effectively convert a MovieClip to Sprite (I’m guessing casting to sprite won’t help much) or another light image type?