Not really. It would only require the new user on the new instance to be able to "own" posts and comments made by the old user on the old instance.

For example, the old user account could transfers its posting and commenting history to the new account (and the new account would be asked to accept the transfer of course).

When it's done, whoever visits the new account, will see posts and comments made from the old account up until the transfer, and the old account's posting and commenting history would be blank.

Then If the old account were to continue posting afterwards for some odd reason, it would build a new posting and commenting history from that point on. Or the transfer could become effective only after the old account is deleted permanently.

I don't know exactly how any of this is implemented, but it would definitely not require monkeying with the actual past posts and comments.