[bsder]

Is there any reason we can't repurpose these lines for HVDC?

It seems like we could get quite a bit of extra juice by changing to DC transmission.

[pyrale]

Same reason you don't repurpose a train into an airplane, the two have different benefits and drawbacks, and are used for different purposes.

[bsder]

Copper wires are copper wires. There are 3 of them for 3 phases. They are sized for AC and AC losses.

DC does not require 3 phases. DC does not have any skin effect or corona losses and thus can operate on smaller wires.

The problem, of course, is that HVDC requires a DC terminal which costs money. It is likely quite a bit easier to build two HVDC terminal on land that the electric company likely already owns than rip up and rewire a whole boatload of long distance wiring.