Yes, anytime during your subscription, you can upgrade or downgrade your plan.
If your subscription page looks like this:
Then if you choose to downgrade or upgrade your plan, you will be automatically moved into that plan right away.
If your pricing page reflects differently then:
If you choose to downgrade your plan, your plan will still reflect the current plan until the end of your billing cycle and it will then automatically downgrade you to the lower plan you selected.
If you choose to upgrade your plan, you will be charged a pro-rated amount based on your new plan rate and number of days left of your billing cycle. As soon as you upgrade, your billing cycle date will reset to that day of the month.