Imagine I have an enormous mesh (millions of polygons) that has UVs and a color map. I'd like to retopo it to something more manageable and bake a normal map, but then I'd also like to transfer the texture, either using the existing UV map or a new one, whichever works best. In principle I think I understand how to do this, but the devil is usually in the details, so if anyone knows of a tutorial that covers these things in some depth or has some hands-on experience, I'd very much appreciate any info.

When you bake the hi-poly mesh to the low poly, you will have the option to bake any or all the maps you wish.  In other words, you can bake a normal map from the details contained in the high poly mesh, but you will also be able to bake textures and color information over to the low poly mesh.  So you can bake diffuse, albedo, specular, glossiness, etc.  This will result in new maps being created for all those shader nodes.  You will not be able to use the existing UV map from the high poly, unless your low poly is unwrapped exactly the same way, which is unlikely to ever be the case.  Baking the info into new texture maps is the easier and better way to go.

Oh cool, thanks! I didn't realize it was that easy. Using a different UV map from the original is not a problem for my use case.

Here's a quick rundown of how to bake color info and maps to a new model.  Not exactly the way you might be doing it, but it should give you perspective on the methods you'll be using.

https://youtu.be/o0lTz25xcx0

Thanks! Yeah, I knew how to bake procedural textures onto UV mapped objects. That's how I got a barbell with procedural knurling from Blender into Poser, as witnessed by my latest few gallery submissions. ;-)

Seeing as I'm new to modern Blender, though, it was more a matter of how to put all the necessary steps together without messing  stuff up. I ended up going with this video which covers everything I needed in great detail, and it still took me half a dozen tries to get it right.

https://www.youtube.com/watch?v=Yx9TvvnxCAM

If you think you might do some texture baking in the future, you might want to have a look at a really good addon for Blender called "Simple Bake".  It really does simplify the baking process to just a few clicks, while creating decent Principled shader structures and PBR maps for you.  If you don't think you'll be doing much baking in the future, then there's really no need for it.  Glad you got it sorted out.

Personally, I do a ton of texture baking, but rarely from within Blender.  I tend to leverage most of my baking through Substance Painter or 3dCoat.  Blender can do a really great job though, once you figure it out.  That's where something like Simple Bake could be a time saver.

Thanks for the tip! I'll see how I go in the future. At this point I really have no idea how much mesh baking I will do.