You don't want to cancel the MA policy, until you have the FL coverage in place. You'll need to mail your MA plates back to the DMV, or your agent, to get the "plate receipt" you need to cancel your MA policy. MA is particularly weird about the documentation you need to cancel your policy.
It will cost you, a "short rate" penalty, to cancel your policy mid term. It's about a month's premium. Can't be avoided, though, if you're moving to FL.