W2s haven't been used in service since the 80s or early 90s I believe, long before the City Circle was born. The typical "City Circle" tram is the last shape of W class tram, the W6/SW6 or W7 (formerly SW5, but they have all since been retired). The W8 class is merely a refurbishment rather than a new tram type.
Bendigo has posted a timelapse of one of the W8 rebuilds. It's pretty much a new tram in the shape of the old, not much of the original tram gets retained.
With a modern tram (and the W8) what ever the pantograph picks up is feed into a traction converter and an auxiliary converter. As long as that converter is 600/750 compatible (and it should be, designing for both isn't difficult in this day and age), it really doesn't matter what the line voltage is.
Of more interest is what happens when a tram designed for 750v goes into regenerative braking and what effect it's attempt to push the line voltage into the 900v region would have on 600v 'heritage' rolling stock.