I like the write-up of this idea. It's well presented. But I'd change one aspect: "We could leave author: 'dril' in the JSON but this is unnecessary too." - kind of. What the post lacks is the record of the identity at the time. What the user's username and the avatar was at the time can change the meaning of the post entirely. To really preserve the message, you need to reference what the displayed identity was used to post it - not just the account id.

There's a number of famous accounts that do it continuously. For example popehat today is "Fucking Bitch Hat" but will change to something else soon that may be related to the current events.