Do I need to reconstruct the comp, manually scaling everything down, piece by piece, or can I just drop the whole comp into my main timeline and scale it all as a comp?
Depends on what is used in the project. AE-internal items (text, shape layers) as well as vector elements can be scaled losslessly by simply relying on the Continuously Rasterize/ Collapse Transformations switch and nesting them. It may interfere with 3D items and a few otehr things, though. For external footage items you have to differentiate: Still images and progressive footage can be scaled easily, but interlaced footage would require to be deinterlaced before scaling. It is also a matter of whether those sources are compressed and what details they contain. Compression may require additional treatments to reduce artifacts, fine details may require special scaling plug-ins so they don't get lost. Effects are an ambiguous matter. Many of them will require finetuning and adjusting their values, others will not. Things like glows and blurs usually look weird when simply scaling their result, effects that produce fine lines (Grid, Trapcode Shine, Knoll Light Factory etc.) and points (CC Particle World, Trapcode Particular etc.) may similarly need adjustments to not lose those details. Other effects that simpyl are used for color corrections and so on will need no adjustments. As I said, it's all down to what you are actually doing...
It's mostly stuff created in after effects. lines, and text and stuff. that makes sense about de-interlacing video and manully scaling effects. Thanks for the