Just to clarify, it's not possible for an AFL team to bring in new people partway through a season. The end of the season (October through to December) is the only time they are able to add people to their playing list. They can negotiate before then, but changes only take place in that window.
Teams can drop players at any time, but they can't add new ones. Instead, they have what is called a rookie list. That's a list of players who are able to be elevated to the senior team at any time, but only to replace someone who has either been taken off the senior list or has been placed on a long-term injury list. Again, players can only be added to the rookie list during that same October-December window.
This is why recruiting has become such a big thing for the AFL clubs. They only get a limited chance to get it right, and then they have to live with those decisions for the entire season before they can bring in anyone new.