Keep in mind that it is possible that someone could have stolen or bought Lafu's account and posted that message so that they could use the new address to prove that the account is theirs, instead of the using the old address that they do not control.

Likewise, if the private key for the old address is found, then the finder could claim that the account was stolen and use the old address to prove they are the original owner.

So, if you need the current owner of Lafu to prove that they are the original owner, you should insist that they sign with the old address.

The solution to the issue is to sign the address change message using the old address.