I'd have to assume that nobody would be asking if the bike didn't originally have internal routing, since that's not something that's possible unless provided for in the frame.

However not all internally routed frames are equally user friendly when it comes to threading a replacement. I've seen some that are so bad that it's just about impossible to thread wires without removing the bottom bracket, so you can make the turn. A few I've worked on were so bad, that I had to thread from both ends to the BB, then attach the gear wire to the "fish" I threaded from the chain stay and pull it the rest of the way.

These days, when a friend asks for help cabling an internal cable, I tell him not to remove the original. I cut it off at the handlebar end, use a length of heat shrink tubing to splice the two together, then --- very gently --- ease the new wire into place. The key is to push and pull together so you don't pull the splice apart.