Here's how I've been working it so far while experimenting on Dennis Bailey's Enterprise A:
Color Layer node: Copy all Color layers from the Standard material that came with the old object.
Scalar Layer node 1: Copy all Diffuse layers from Standard material. Plug this into Roughness.
Scalar Layer node 2: Copy all Spec layers and plug into Sheen.
I'm still figuring things out as I go but I've gotten some nice looking results so far. Now, if I can figure out how to get the diffuse maps to act the same way with the Principled shader then I'll be a happy man.
There are some things which the BSDF shader doesn't work well for and one of those things is large planets. You can't control the diffuse sharpness like you can with Standard (at least that I've found). It's great for "smaller" objects where material can make a difference (like cars, asteroids, buildings, bugs), but on a planetary scale I've seen no benefit.