No problem at all. You can downgrade (e.g. weekly > biweekly) or upgrade (e.g. biweekly > weekly) at any point. Simply let me know during one of your lessons or by sending me a message or email, and I'll change or cancel your subscription accordingly.
I do ask that you let me know at least 10 days before the next month starts. However, I'm a decent human being so I won't let you pay for another month if you don't actually want chess coaching. So please don't worry about this being a subscription, it's just for the purpose of the payments being automated - neither you nor I have to think about it again.