It is easy to change the membership plan from week after week to month to month or yearly or the other way around. You can adhere to the directions on the Apple site.
Apple handles the effect of such changes. When you change the plan, the 'change' will be effective as per your new preferred frequency after completing the existing cycle. That is, if your existing plan is monthly and 5 days are still remaining for the subscription to renew and you switch to the yearly plan, the yearly plan will be in effect when the current subscription month completes - after 5 days.
In other words, the change in plan will not happen immediately, if there are still a few days remaining for the existing subscription cycle to complete. In such a situation, after those remaining days are passed, the new subscription cycle will be in effect.