There is a lot more study that has to be done to map out any sort of evolutionary path for this change. The team is looking more closely at the "new" genes in Volvox, as well as comparing both species to a third, Gonium, which is an intermediate between Volvox and Chlamydomonas. It's one of the first breakthroughs in determining how sexual dimorphism originally around. It will take time, but it's a very interesting first step to solving this evolutionary mystery.
Source: Science Daily- Lessons from the Pond: Clues from green algae on the origin of males and females