Once a copy of a web site has been made with --mirror, it would be useful to have a --mirror-update option that could be used to cause wget to check and update the mirror copy as necessary.

This is rather messy to do.  Among other things, wget would need to maintain a status file in which the mapping from URLs to local files is kept, and the entries in it need to be retained forever (in case a previously-deleted URL becomes live again and some local page's converted link to that URL remains).

Given that such a status file is needed, it might as well be used to log "work yet to be done" in the update and periodically be written to disk to checkpoint the wget run.

Related tracker items are:
http://savannah.gnu.org/bugs/?49226  Add possibility to append to existing WARC file
http://savannah.gnu.org/bugs/?34415  Add option to delete local files/directories if they do not exist on the server
http://savannah.gnu.org/bugs/?33372  Quick resume for mirroring operation.
https://savannah.gnu.org/bugs/?25340  --mirror and --convert-links mixing poorly?