It's good practice to re-use as many assets as you can, in this case that logic definitely applies. if you can get away with just re-optimizing an existing mesh and rebaking the existing textures, the go for it.
You may need to do some manual cleanup, as not everything will bake over properly (you should definitely rebake the normals from the original highpoly btw). But that will still save time vs doing the whole lot from scratch, right?
Rebaking normals from the low, if your normals are synced up properly isn't a bad idea. You're more likely to have all of the details line up if you bake normals, diffuse, spec, and gloss from the lowpoly than if you bake the normals from the high (you may get projection skewing/errors with a lower resolution third person view mesh).
Just make sure your baker can actually bake a low+normals as a proper normal map, you can't bake it out as if it was a diffuse map, that won't work.
Also, it isn't uncommon to simply reduce the model and use the same texture sheet. If you rebake the maps it will look better, but often you won't see a TPV asset up close so minor errors do not matter. When you do your uvs you should think about how easily you can remove edge loops while maintaining the same UV layout. Usually optimization is heavily reliant on the UV layout.
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You may need to do some manual cleanup, as not everything will bake over properly (you should definitely rebake the normals from the original highpoly btw). But that will still save time vs doing the whole lot from scratch, right?
Just make sure your baker can actually bake a low+normals as a proper normal map, you can't bake it out as if it was a diffuse map, that won't work.
Also, it isn't uncommon to simply reduce the model and use the same texture sheet. If you rebake the maps it will look better, but often you won't see a TPV asset up close so minor errors do not matter. When you do your uvs you should think about how easily you can remove edge loops while maintaining the same UV layout. Usually optimization is heavily reliant on the UV layout.