Yeah, if putting in Clients user/password, apart from validating it, as you may need to answer his security/validation questions, thats it,  there is a button below  when setting up to login in, rather than creating a new account..   if that makes sense..

when trying to connect to existing PP account, it will recognise that you are trying to sign in from a different computer/location and will ask for security..  easiest if done over the phone at the same time...Obviously needs to trust you,  and then once all connected,  he can just change his password to keep his account secure..

As @Stefano says, If End user changes password, you will not need to re-link..

@ Andy A. & @ Stefano

All clear now - thx!