You can change your plan at any time. Right now the only differences between the plans are cost and billing frequency. If you'd like to go from monthly payment to yearly to take advantage of the discount, or the other way around (to take advantage of... paying more?) you can!
Click on your username in the top right corner, then click "Edit Studio". In the first tab, entitled "Account" you'll see a big red button labeled "Change Plan". Click on that and then confirm.
If you're going from monthly to yearly then your plan will be updated immediately and you'll be billed a prorated amount based on the credit you have left that month.
If you're going from yearly to monthly then your plan change will take effect at the end of the plan period.