Xbox Ultimate and Xbox Gold

So I purchased a year long xbox live gold membership back in March of this year and it expires of March next year. For me right now I have 10$ in my account and Xbox Ultimate is only 1$ for 1 month. I want to get the Ultimate for 1 month then cancel it
so I dont have to pay the 15$ every month. But I have 2 questions. It says if I purchase Ultimate it will upgrade my remaining membership. So does that mean that since my Xbox Live gold runs out in March then the Ultimate will start making me pay 15$ in March?
Or will it make me start paying 15$ after the one month is up. My next question is if I do want to cancel my Ultimate early will it also cancel my gold that ends in March or does it only cancel Ultimate? Thanks to anyone who sees and answers this

:)