Your Master Account in Yahoo functions similarly to the Master Client Center in AdWords.
Whenever you join an account to your Master Account, you gain administrative and editorial control to these accounts. You then have the option to log in as your master account ID, the original account ID, or you can create a new user and give this ID access to the joined account.
Once the account is joined you can access their campaigns, data, run reports, and pursue all the functionality of these linked accounts as if you created the account yourself. You can set certain login IDs to “manage” specific accounts and give roles to them related to the account in question.
However, all things in the Search Marketing world (and in life) are temporal, you may find that eventually your relationship with a client ends, and you no longer need, want, or should be able to access their PPC account on Yahoo anymore. That is very unfortunate for your client because there is no way to unlink an account short of deletion and creating an entirely new account. Beyond that, our Yahoo rep actually said that they suggest this course of action.
I was assured me that there was no other options and nothing was in the pipeline to allow accounts to be unlinked.
I asked if the account would take a quality score hit for losing all its historical data, and he said “Probably, in the short term.”
There you have it. Linking accounts in YSM is til death do us part. Be warned!