when this ban is lifted in '04, will it downgrade ALL class 3 weapons to regular long rifle status, or just SOME, or will it just allow you to customize your AR more?  I read the ban, but I'm just not sure what all of the legal interpretations result in.

AWB has nothing to do with class 3 weapons.

Machine guns and other NFA weapons do not fall under the '94 AWB.

When the AWB sunsets and assuming there are no replacements for it, you can put more than 1 evil feature on your post-1994 semi-automatic firearm.  A post-ban like the Bushmaster can then have a collapsible stock, flash hider (and suppressors/silencers), grenade launcher and bayo lug installed. The import ban of 1990 is not affected.  You will still not be able to convert those old post 1990 thumbhole stock imports to AW configuration unless there are enough US parts in them.

Machine guns would not be affected by the sunset at all.  Post-ban registered short-barrelled rifles previously affected by the AWB could have the various evil features added. The US parts count does not apply to NFA firearms, though.