Not sure about other people, but I still haven't received any letter nor is there any notice when I logon to chase.com my card being switched over to the MasterCard.
When I secure messaged chase, I was told:
We do not have an exact date when the account will
transition from Visa to Mastercard. However, you new card
will be mailed in April 2014 or May 2014.
So those who haven't been switched over already, you may have a chance to cancel still before everyone who is remaining is migrated over so as to preserve your eligibility for the IHG MasterCard bonus.