[nickwarren]

As I understand it, reconductoring main benefit is it allows you to fit more aluminum in the area. NEC specifies conduit has acceptable % fill value that I could see this optimization having for retrofits, but long haul transmission lines are not underground - they're dangling in the air where it doesn't seem material if the wire is a bit bigger on replacement.

Am I missing something?

[devanl]

As hinted at, but not elaborated on by the first part that mentions temperature monitoring, those wires in free air expand and contract with temperature changes. The cables need to be able to support their own weight without collapsing or touching other conductive objects like other cables or trees.

Adding more conductive material helps with the resistive losses, but also makes the cable heavier. Using an advanced cable that has better tensile strength for the same or better conductivity accounts for those second order effects better than just adding more aluminum.