Cut the cable, what now for the coax?

[Angrboda]

Buying an adapter doesn't seem to make sense.  You can get 50' of ethernet cable for $10 and that's simpler and doesn't require an adapter.

Donate that shit to Goodwill.

The coax is already in place, and I'm not flexible enough to run new line. The lan cable would have to go through the floor in the office, through the walls in the basement twice and then up into the living room. As I don't want to put more holes in the floor I'd have to move two large and fully loaded bookcases to get to the same hole.

Fair enough. Do you have bandwidth needs that aren't met by WiFi?