So why was the shape of Zyra's Q changed?

Hmm that would make sense. I had thought previously as well that it was to make it a bit harder to hit vs champs because her plants got buffed.

I agree with MotherTeresa, that's probably the reason for this, since seeds can still only turn into plants by using a skill over them and Zyra sometimes had a dilemma of choosing between using the Q for damage or strategical plants spawn. This way there will be more "accidental" or additional spawns, otherwise the passive would be wasted.