If it is standard CATv cable and not shielded then as you say its probably not much different to running the cable adjacent to each other.
Its not ideal, but there are other options...
1. have the modem and the router downstairs, then just have a little switch upstairs connected to ya existing CATv cable.
2. have just the modem downstairs and then the router upstairs, use the CATv for connecting the modem to the router, and then maybe get a powerline adapter to link upstairs to downstairs.
But I would just install another cable, its not rocket surgery its just being arsed to do it!
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