The reason I'm thinking Harmon as an up safety is that it was his primary role in his first three seasons, while DMac was the deep man (and a great one). There was some switching off in 2015, and then more Harmon in the back and more DMac roaming in 2016. I think that was tied in with the Pats incorporating more pattern-matching in their coverage schemes.
If Chung is out, I really think Harmon moves up, JMac takes more of a deep role and DMac continues in his "roaming" role. But I'm hesitant to pigeonhole anybody, really, since the Pats coverage schemes are so fluid and varied.
And, there are some variables that could lead to more 3-CB nickel and less 3-safety nickel this season, even if everyone is healthy.