The plan_id is not changed even after update subscription.
Why is this?
Also, what should I do if I want to change an active subscription to a different plan_id?
Currently the ability to update the plan_id for a subscription isn’t currently available. You’ll need to cancel the existing subscription and create a new one with the new plan_id. 
I understood.
I consulted the reference and found a feature called SWAP_PLAN, can I use it to do what I want to do?
Yes, so sorry. That just got released and you can definitely use that to update swap the plan.
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