Hi Ben,
Glad to hear it shouldn't be a breaking change.
So in principle, if we don't make any changes our 3rd party applications, that currently use category information, will provide the reduced set of over-arching categories. We'll need to make changes to our code if we wish to expose the additional sub-category information. Both the category (place_category_id) and sub-category (presumably something like place_subcategory_id) will come back on any API call requesting place information.
I know you can't make a blanket guarantee, as you don't know how each developer has written their specific code, so there is an obvious assumption that we haven't used any techniques to hard code or cache category information.
Thanks,
Jonathan
@mobijack