Not a lawyer here either but I'd say you're safe if your visa has already been granted. By grandfathering of the existing visas, they probably mean that there will be a path for existing 457s to move on to another visa when their term expires so usually either a residency or citizenship. They did this with other visas before(family visas) where existing(and in progress) ones remained until expiry but newer ones were not granted.

The 457 system, similar to the student visa before it, was being abused by companies. So the government has been working on fixing the loopholes in the system for quite a while now.