Plans templates are denormalized
This means that when you assign a plan to a person it will (behind the scenes) create a copy of this plan with the attributes that plan has at that instance and assign to that person. When you update the main plan template it will not affect or change people who have already been assigned that plan. (unless you choose to update subscriptions for any change)
A few examples...
Raising plan cost
Say you want to raise the price on a plan for everyone already on it. If you go into the plan template and raise the price it will not change the plan price for everyone who is already on the plan unless you turn on toggle to update subscription.
Change sessions on a limited plan
It's time to adjust the number of sessions on your limited checkin plan. If you go into the plan template and change the number of sessions it will not change the number available for everyone who is already on the plan unless you turn on toggle to update subscription.
Changing any other plan setting/attribute
See paragraph above.
Deleting a Plan
When you delete a plan, it will not affect the members already on that plan, they will continue to be billed if its a recurring plan, and that plan will remain active on their account until you cancel it on their member detail page.
What if you want to update the Plan Amount and have those changes cascade down to all members already on the plan?
You can do this too. When you update the plan amount attribute you will notice a toggle appears with this option. Turn either of these on to update existing members with this plan to current settings of plan.
The only thing that isn't denormalized on plans is the tax rate. if you add tax to the base plan it will start adding tax to future payments.