"Updates to your Creative Cloud Photography plan"

I got the following email:

"The price of the Photography plan (20GB) annual, billed monthly plan is changing from US$9.99/month to US$14.99/month (plus applicable taxes) on your next renewal date. To maintain your effective current base rate (excluding any promotions) you can change your subscription to an annual, prepaid plan. 

Regrettably, this option is unavailable.

When I attempt to “manage my plan,” the only available option is “Cancel your plan.”

Where is the option to prepay annually? In fact, it seems there is no way to “maintain my effective current base rate.”