I can only see positives.
Zorko would offer more run, and defence than Birchall. Coleman and Starcevich are both good defensively. Really the only one of the seven defenders who might be considered a weak defender is Rich.
As for overhead marks being a weakness of Zorko, that really only becomes an issue if he’s against a much taller opponent. Most teams these days play multiple small forwards, so having another small defender to partner Coleman is a good thing. And from memory, the vast majority of Birchall’s marks in the defensive 50 were largely uncontested, so you would hope for the same with Zorko.
