First you have to cancel the one you have right now. If you do so, you can choose from two options:
Cancel Right Away
Cancel After Expiration
If you Cancel Right Away, then your current plan is immediately canceled and you will
be redirected so you can purchase the upgraded one and start it right away, but you will lose the time
you have left on your current one.
If you Cancel After Expiration, then you can finish your current subscription, the
system will automatically cancel it after the expiration date and you are free to upgrade afterwards.