There has been no suggestion yet that existing accounts will change. I think they would lose a lot of existing customers if they did it any time soon.
It seems to me that they have been told to harmonise their offering with MailChimp - possibly to avoid people using Mandrill as a cheap MailChimp substitute? Just a guess on my part.
Actually, if you're sending 250,000 emails per month, I think $50 per month is actually quite cheap!