Strictly speaking, I don't think you can "renew" in advance. "Renewal" really only occurs at the end of your term.
You can sign up for a one-year plan anytime. Your "leftover" amount from your previous plan are applied towards your new one year plan so you actually end up paying less than the full one year amount.
Note that when you do this, you actually end up signing up for a new plan so you'll be changed to the Happy Hosting plan from your original Crazy Domain Insane plan (or whatever plan you were on).
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