i like it. if i'm not mistaken, youtube videos of old entries follow this logic :)
i agree with user-toggling of this if it's implemented, but if we wanted to take it even a step further, each user could enable/disable the use of each individual one of their prior avatars. if an avatar is disabled for historical use, a post/entry could display the next avatar (forward in time) that is enabled. and disabled old avatars would be hidden from the avatar display on your profile, for anyone but you and admins.