Hey Armando,
I need to keep the old subscription id remain, why don't you change only the plan id for the same old subscription?
I am sorry to say but it's not possible to upgrade plan on same subscription id at the moment in the system. This is not just the change the plan id, it will require each and every entity related to subscription need to update which is not possible. Upgrade always create separate new subscription.
This is due I have a proprietary system integrated with Payplan and don't work for me if every time a subscription is upgraded move also the subscription id in the database, this action loose the tracking I have in my system.
Can you exaplin me how you track the system , so i'll help you to assist on this.
In your tracking system you just save the subscription id or ?
Looking foreword to your response.