Counter-intuitively, if you do not want the account uuid of the user to change, the "new" account should be the pre-existing account, and the "old" account should be the redundant second account that was more recently created. This means "old" and "new" are opposite from their expected chronological meaning. In this case, the use of "old" and "new" reflect the direction of transfer of ownership -- the login was associated with the "old" user account, but will be associated with the "new" user account.
Counter-intuitively, if you do not want the account uuid of the user to change, the "new" account should be the pre-existing account, and the "old" account should be the redundant second account that was more recently created. This means "old" and "new" are opposite from their expected chronological meaning. In this case, the use of "old" and "new" reflect the direction of transfer of ownership -- the login was associated with the "old" user account, but will be associated with the "new" user account.