I just got this 1985 mountain Klein frame off craigslist and am building it up. It has internal cables with nylon housing inside the frame. I'm trying to replace the nylon housing since its 30 years old but it's attached inside the frame by the head tube. The ends by the chainstays are free. How would I go about detaching it without damaging the frame and when I put the new housing in do I need to fix it to the frame? I thought about just running full shift cable through the whole frame but The holes were too small and I'm not about to start drilling holes in the frame.

The cable routing is the same as on this bike. Mountain Klein 1985- Mtbr.com