Save(someValue) is for the case where you want the resulting record to be or become the full object (someValue) you passed in.
You can use
var query = Query.EQ("_id","123");
var sortBy = SortBy.Null;
var update = Update.Inc("LoginCount",1).Set("LastLogin",DateTime.UtcNow); // some update, you can chain a series of update commands here
Using FindAndModify you can specify exactly which fields in an existing record to change and leave the rest alone.
You can see an example here.
The only thing you need from the existing record would be its _id, the 2 secret fields need not be loaded or ever mapped back into your POCO object.