$merge whenMatched pipeline

Working with Data
1 
const mergePipeline = [
    {'$unset': "_id"},
    {'$addFields' : {"Status" : "New"}},
    {'$merge' : {
        into: "previous",
        on: "Role ID",
        whenMatched: [
            // compare the documents with $cmp <-- is it possible to only compare a few fields without unsetting them?
            // if different replace root with "new" document
            // change status to "updated"
        ],
        whenNotMatched: "insert"
    }}
];

db.current.aggregate(mergePipeline);

I have been looking at $merge and Variables in Aggregation Expressions but I am struggling to understand. What I would like to do in a very general sense is take two collections, match them on the unique “Role ID” field and see if they are exactly the same or not. If they are the same I want to update the “Status” field to “Updated”.

Where I am struggling is on the whenMatched pipeline. I am not sure how to target the “new” and “old” document for the $cmp expression. I am also not tied to this approach. I feel like $mergeObjects could be used as well. I appreciate the help.