"Getting a status match is very easy. Send an email to StatusMatchMilleMiglia@alitalia.com by December 31, 2015 with the following:
First name and last name
MilleMiglia code
A legible copy of your existing valid Frequent Flyer program card
Your status will be effective 8 working days after you send the request email and will be maintained until March 31, 2016. To confirm that your status has been recognized, you will receive a personalized email from your new Club. If the status in the existing Frequent Flyer Program expires after March 31, 2016, the equivalent status recognized within the MilleMiglia Program will be valid until March 31, 2017."
For further information see the official Alitalia website
http://www.alitalia.com/en_en/millemiglia/promo-millemiglia/promo-status-match.html.
My status changed online about a week after I sent the email. Card arrived very fast within 2 weeks!