Ive just got an email from MM. Miles are valid until March 31st 2018.

It also says that if one takes 4 AZ flights between January 1st and march 1st 2018 one can keep up to 100% of its current mileage balance for the new program starting April 1st.

Not sure if it's related but as i'm trying to burn 100k miles I can find longhaul reaward tickets availability even in business where few weeks ago no award tickets were available on AZ in any class.