Purchasing rental property with existing Sec 8 Tenant

We're attempting to purchase a rental property in Marietta, GA that currently has a Sec 8 tenant in place until July 2019. The tenants intend to continue leasing, and our intent is to continue renting it them. My question is what sort of paperwork do we as the new landlords need to file with the PHA in order to receive the montly rental payments. I went on the Atlanta PHA website, and it mentions the "HAP Assignment Contract Document" that needs to be certified between old and new landlords. Is that it? And is it typically part of the closing process or does the transfer happen after closing (and how long does it normally take)? Trying to figure out the most efficient way to do all this and ensure a seamless (ie minimal pay interruption) transition. 

Source: http://www.mariettahousingauthority.org/index.php/...