The problem is the many client-side exploits on the old client. I don't know to what extent and how easily they could apply the changes from the modern client. 2004 computer engineering is very different than 2017.
I guess they will take the most automatable solution. The one with the less work by hand, but which one is it ?
To know which road they took is the first question I would ask to Blizzard if I could.