Compare two collections and update or insert as needed using mongodb aggregation pipeline

Question

I have a previous and a current collection. The collections have the following fields: userID, username, lastPosted. I want to update the previous collection if anything has changed or insert any "missing" documents. Missing being documents in current by not previous.

I was looking at this answer and it got me to a point where I could find all the deltas. I think the next steps would be to grab the userIDs from the deltaCollection and merge them into the previousCollection.

    db.getCollection("previousCollection").aggregate([
          { '$unset': "_id" },
          { '$project': {
                  'from': "previousCollection", 'doc': "$$ROOT" }
          },
         { '$unionWith':
          {'coll': "currentCollection",
             'pipeline': [
                  { '$unset': "_id" },
                  { '$project': { 'from': "currentCollection", 'doc': "$$ROOT" } }
              ]
         }
          },
          { '$group':
              {'_id': "$doc",
          'previous': { '$sum': { '$cond': [ { '$eq': ["$from", "previousCollection"] }, 1, 0 ] } },
          'current': { '$sum': { '$cond': [ { '$eq': ["$from", "currentCollection"] }, 1, 0 ] } }   }},
          { '$match': { '$expr': { '$ne': ["$previous", "$current"] } } },
          { '$match': { "current": {'$eq':1}}},
          {'$unset': ["current", "previous"]},
          {$out: "deltaCollection"}
      ], {allowDiskUse: true});

What I want to be able to do is update/insert documents that need to be worked on. I think I need to do something with $mergeObjects or $replaceRoot but I am not really sure. I feel like I should be able to group the two collections on the userID and then go from there.