<snip> at the same time), then they would be able
to keep the old wires since higher voltage reduces the need for higher
All the current infrastructure is EOL this is why it is being changed.
current to provide the same power. Of course swapping all that out at once is not an option, so the voltage is not changing. So instead if you want more power capacity, you need more current which means thicker wires. Conviniently that can be done area by area.
The wires are of course also being updated to work with the pantograph system, which means fixing all the intersections and curves, and eventually making the straight sections move side to side a bit rather than being completely straight (which apparently wears a nice groove into pantographs). After all the plan apparently is that only the first 60 or so of the new streetcars will have trolley poles, while the rest will only have a pantograph (the first 60 have both).
From Wikipedia bit on transport pantograph
"Pantographs with overhead wires are now the dominant form of current collection for modern electric trains because, although more fragile than a third-rail system, they allow the use of higher voltages."
-- Len Sorensen