In discussion on the discord general consensus is that users want Avatar Scaling Enabled by default. The announcement states that the default for worlds is for Scaling to be disabled.
As a compromise I think it'd be a good idea to have Scaling ON by default for NEW WORLDS going forward as a qol improvement, even if just in the upload UI for the world in unity, while grandfathering OLD WORLDS into the system with Scaling turned OFF to make sure those worlds retain their previous behavior as per the 'Why is Scaling off by default?' question in the FAQ for the beta post. Any old worlds that still have active developers can still turn Scaling on at their own discretion and New worlds turn it off.
This behavior would be similar to how Physbones 1.0 to 1.1 is handled. Old Physbone are 1.0 while new physbone components are defaulted to 1.1. While not exactly comparable, I think this situation could likely be handled in a similar fashion.