Are you using a parent > child relationship? If going from a parent plan to then having a child plan they'll always have the parent plan. When upgrading from 1 Exclusive plan to another Exclusive plan though the previous plan will remain active until it truly expires. So if you unsubscribe from the new plan while the previous plan has yet to expire then you'll still have the previous plan as active. As far as I am aware there is no option to do otherwise. I suppose you could use CBSubs SQL Actions to query the database and expire the previous subscription as previous subscription id substitution is available.

I see, it's working as intended then. There is no feature to disable this usage. It's appropriate to pause the previous plan when using the upgraded plan and if the previous plan is not yet expired when cancelling/unsubscribing from the upgraded plan it'll revert to the previous as active.

Could you please let us know the exact file and function to be modified to achieve it?

No, we do not support core edits. It also would not be a simple edit.

It would be great if you guide us in this scenario with some instruction to deactivate previous plan where they upgraded from when they cancel current membership.

Your only solution is to use CBSubs SQL Actions to unsubscribe the previous plan when upgrading through database queries. Example query as follows to unsubscribe previous plan id (not this is just an example and was not tested).