New option --mode=[insert|upsert|merge] defaults to insert. The distinction between the options is a follows:

• insert: always inserts a new document
• upsert: replace existing documents, or insert otherwise (relies on --upsertFields)
• merge: modify existing documents, or insert otherwise (relies on --upsertFields). This preserves values that are not overwritten.

Use of --upsert is deprecated. For now, it aliases to --mode=upsert.