So after changing my username once, I looked around and realized some things that reference your username don't change. This mainly involves the forums, where the last sent messages will show your old username, as well as the edited message. I don't know if this is intentional, but here I am.

It's not exactly intentional. It's just how the Forums are structured and it significantly predated Username changes. There isn't much I can do until I eventually go into the code itself for the Forums and update how it reads Usernames.