It’s worth remembering a few things when adding another plan to your account:
you must always have a plan on your account without a discount. This is known as your qualifying plan
if you cancel your qualifying plan, the 10% saving will be removed from the oldest plan on your account. Or from the most expensive if two or more of your oldest accounts started on the same date
the discount applies to your monthly plan, not any other charges
any price increases will be based on the full cost of your plan