After a user changes their avatar, the image file still gets stored in the phpbb avatars folder.
The image can only be found if the user has a direct link to it. It cannot be re linked to the user account.
I suggest you create a "Personal Avatar Gallery" and list all the previous avatars to allow the user to permanently delete the file or to re use it if they wish to, without re uploading the same image.
The bellow first link is my first avatar and the second is the one i just updated. They can be listed in the UCP by using their id.