When I made my post above some 11 months ago (!), I was relying on an experienced auto parts (generic) specialist to inform my post. Indeed, most roof rails either have a punch out or a fitting the attaching bolt slides into.
Since then I purchased an LX and promptly deleted the roof rails, hopefully gaining back that lost mpg. The LX has threaded studs welded permanently to the roof. Not having the FE roof trim pieces, I thought those studs were specific to the LX (and above) and that they protruded too high to allow the FE roof trim to fit (now I'm not so sure). I had originally hoped to swap the roof rails with an FE owner, but decided that wasn't possible. So I immediately took the studs down with a Dremel - it was clear that all Niro's could receive the roof trim as the welded fitments for them were on my LX and knew I'd never ever use the rails.
I delayed fitting the roof trim for some time as the dealer parts could not find a number for them in their system. I thought I was going to have to find a junk yard FE to get them. However an auto body shop I was dealing with was able to pull the part number and order them from that same dealer! So my car now looks like an FE to observers.
Now I'm thinking that it only makes sense from a manufacturer perspective not to set up welding robots for two different models.
Easy for an owner to check this hypothesis. Pry up one end of the roof trim with something plastic (perhaps two credit cards or a bicycle nylon tire tool). Then pull it up a bit further (or the whole way - easy) and see if you see any threaded studs. If they are there, you can go for the OEM rails!
And I'd be happy to sell you my rails (I'll include the special deep socket required for the nuts). But do let us know what you find out.
2018 Kia LX HEV Metal Stream with Advanced Tech