I too, just relieved this ominous storage reduction email today. I will say that I think it is BULLS#it, I was never made aware of any opt-in option, and just did a search of all my emails, and the only thing I found was back on 2/25/16 alerting me that my 365 was going from unlimited to 1TB of storage, but nothing anywhere regarding this issue today about going from a total of 30GB to a net of 5GB. I don't make it a point to follow every aspect of trusted business's comings and goings, for I have other things to do...like taking care of my own customers.
I was taught to stand by your word. When you make a deal, you stand by it...PERIOD! You CAN NOT unilaterally change the rules half way through the game...PERIOD! I am sure there is the "small print" somewhere, and this is the part that pisses me off...it should be the "LARGE PRINT". I think that someone is taking advantage of others. I would be more than happy to help be part of a solution, if I was just asked (I have 10TB of promised storage and would be more than happy to give up what I will not use...again, if asked). I just get TRIED of every mismanaged entity thinking that their customers are suppose to suffer/pay for their errors. Roll someone else head, I already dizzy!