Add remaining time when subscribing to another plan in the same package

I have an Overwolf app, and I have a single premium plan in my app, but with different durations (one month, six months, and one year).
They are all part of the same package.
I would love it if when a user buys one of these plans, it would:

• cancel their existing plan (though this is nice to have, it’s optional, as I can force them to unsubscribe first)
• add the time left on their current active (but cancelled plan) to the duration of the new one