Hello. Alexander is right here:

A user must explicitly subscribe to a payment subscription by himself. You can not subscribe him yourself. Otherwise that would be a security problem. So if a user wants to change his subscription the he'd better cancel the existing one and then explicitly subscribe to a different one himself.

You as site admin cannot change the subscription of some other user. Moreover current integration event doesn't allow to do anything like this to subscription owner.

If you update a subscription manually on UNA end Stripe won't know anything about it and continue to charge for the subscription which was initially created.