There is an existing stackoverflow question which answers this question but the call isn't working anymore.
I tried adding http://open.api.ebay.com/Shopping?callname=GetCategoryInfo&appid=YOUR-APP-ID&siteid=3&CategoryID=-1&version=729&IncludeSelector=ChildCategories&DetailLevel=ReturnAll
But it didn't help.
It is still possible to return the entire hierarchy in one call?
To the call but that didn't help.