The major difference about the symbols and instances between the new export and the old one is that, in the old version we were treating the first instance of a symbol as the definition of the symbol and now we are putting all the symbol definitions under the Symbols null and making them invisible in the scene. Other than this, there's not a difference in how they behave.
If you wanted to change something about a group of instances, you can just go change the symbol definition under the Symbols header and all the instances that are linked to it will change as well.
In addition to this, if you wanted to have your symbol definition visible in the scene, you can toggle the small dots on the right side of the object to green and the symbol will be visible at the origin of your model.
Hope this helps