Comment by alienchow
I do have PVC conduits under the flooring. You can see the photos here: https://alienchow.dev/post/homelab1/
In theory I can pull a new cable through. But practically it might be tough due to the number of bends (shelter -> wall -> vent -> ceiling -> wall -> floor -> room). In the worst case scenario I can give it a try, but it's probably going to destroy the new fibre cable when I pull it through. For now the connection still works, so I am hoping it doesn't get to the point where I have to give that a try.
you can always try the plastic bag + vacuum cleaner trick - take a thin flexible rope, tie it to a small plastic bag, stuff the small plastic bag into the conduit, use a vacuum cleaner at the other end to suck the plastic bag & rope through. You can then use the rope to pull through new cable. If you make the rope twice the length of the conduit, you can keep it in there indefinitely to pull through new cable whenever you want.