Owen Duffy wrote in
:

....
....
in the other (defined by differential mode), and V conductor to
conductor is twice V conductor to shield, so V'=2*(I*Zo), and
Zo'=V'/I=2*(I*Zo)/I=2*Zo.

I should have expected that this is not obvious to you Jeff, and expand that
to:

....
in the other (defined by differential mode), and V conductor to
conductor is (by definition of differential mode) V'=(I*Zo-(-I*Zo))=2*I*Zo,
and Zo'=V'/I=2*(I*Zo)/I=2*Zo.

Owen