The article doesn't say much about it but I am sure there is significant work being done at the transmission substations as well to support the extra capacity.
More likely is that lower impedance on the reconductored circuit will cause increased flows on other, non-upgraded circuits, either requiring those to be reconductored, or installing phase-shifting tranformers or reactors to limit current.
Have you seen a lot of phase-shifting transformers in the U.S.? In my experience they've mostly been in Europe with a few specialized applications in the States.
I would think a utility would want to reconductor the other circuits otherwise they're leaving benefits on the table right?
They would love to reconductor the other circuits. In the US, the utilities make a guaranteed rate of return on investments in the transmission system. So, anything they regulators will let them do, they'll do -- not necessarily because it has technical benefits, but because it has economic benefits.
This is one reason why reconductoring isn't that popular with utilities -- it allows the utility to get more capacity with less spend, so less profit.
IIRC, the expert answer was: substations generally need a retrofit (eg new transformer, breakers, smarts).
Even so, reconductoring is much faster and cheaper than building new lines.
Because retro doesn't require a new permit, often reuse existing footprint, and substations can be upgraded as needed (eg only for sections pushing more power).