How to hierarchical to true

It looks like by default hierarchical is set to false. I am wondering if there is a way to set this to true without editing the actual plugin file as I am looking to maintain parent/child structure in the taxonomy permalinks. I tried implementing something like this: https://wordpress.stackexchange.com/questions/161788/how-to-modify-a-taxonomy-thats-already-registered?rq=1 with no luck.