Welcome to DreamHost! I'm glad things are going well, and hope you remain happy with your decision.
Unfortunately, Promo-Codes are only good for new sign-ups, must be entered at sign-up time, and cannot be used for upgrades or renewals. They are designed to "introduce" a new user to DH.
I've found the regular prices are still the best thing going, and you can always help defray your ongoing costs by creating and distributing your own Promo-Codes. Many pay nothing, and make money this way, though it is harder now with all the $97 off codes .