There is a new option in event form view, "Create Partners in registration". If
this option is checked, when you add registrations to this event, partners will
be created automatically with name, email, phone and mobile fields.

If partner already exists and user only fills email, name, phone and mobile fields will
be filled with partner's data.

The event registration values email, name, phone and mobile will be changed if the
related partner values are changed and the event end date hasn't passed yet.