I think it really depends on which bank issued the card and what the account holder and AU value more, the account history or the CL.
If the AU valued the length of the account history more, I would imagine that would be more difficult to preserve in any transfer of accounts because fundamentally, a credit history is a record of credit activity of the person in question. Unless the AU legally assumed the identity of the account holder (I'm not sure if that's even possible), it would be pretty tough to convince a bank that a history assigned to someone is actually supposed to belong to another. All credit decisions made on the account would have been made given the circumstances of the account holder.
However, if the AU want to preserve the CL only, it may be possible to open another joint credit card account with both the AU and the account holder and transfer the limit over from the original card. The limit could be saved but the joint account would be new. The account holder can then either cancel the original card or leave a token limit to preserve the history. This is contingent on the bank allowing transferring of limits of course, but it should be more commonplace than transferring a tradeline history or adding a joint account holder after the fact.
One more thing to note. If the ultimate goal is to achieve sole account ownership while preserving that limit, it may be very difficult. Issuers may be reluctant to remove a joint account owner so that may be as far as this tradeline or line of credit can progress. The easiest thing to do may in fact be to start with a new account and garden from there.
