I can't find the official announcement, though I do remember this information being posted here.
There is certainly a lot of notice on the "Manage Account" page where you change your rebill period:
You can: "Change to our "Happy Hosting" plan?"
or: "Change Rebill Period?
(Automatically upgrades you to our one plan!)"
Unfortunately, it's not like you were actually on the "monthly crazy domain insane plan". You were on the "annual crazy domain insane plan" and changed to the "monthly happy hosting plan".
Maybe if you contact support they can put you back on your old plan? Of course, you'd be back to annual billing...
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