I believe you can use rsync to do this. The key observation would be in needing to use the --existing and --update switches.
--existing skip creating new files on receiver -u, --update skip files that are newer on the receiver
A command like this would do it:
$ rsync -avz --update --existing src/ dst
Example
Say we have the following sample data.
$ mkdir -p src/; touch src/file{1..3} $ mkdir -p dst/; touch dst/file{2..3} $ touch -d 20120101 src/file2
Which looks as follows:
$ ls -l src/ dst/ dst/: total 0 -rw-rw-r--. 1 saml saml 0 Feb 27 01:00 file2 -rw-rw-r--. 1 saml saml 0 Feb 27 01:00 file3 src/: total 0 -rw-rw-r--. 1 saml saml 0 Feb 27 01:00 file1 -rw-rw-r--. 1 saml saml 0 Jan 1 2012 file2 -rw-rw-r--. 1 saml saml 0 Feb 27 01:00 file3
Now if I were to sync these directories nothing would happen:
$ rsync -avz --update --existing src/ dst sending incremental file list sent 12 bytes received 31 bytes 406.00 bytes/sec total size is 0 speedup is 0.00
If we touch a source file so that it's newer:
$ touch src/file3 $ ls -l src/file3 -rw-rw-r--. 1 saml saml 0 Feb 27 01:04 src/file3
Another run of the rsync command:
$ rsync -avz --update --existing src/ dst sending incremental file list file3 sent 115 bytes received 31 bytes 292.00 bytes/sec total size is 0 speedup is 0.00
We can see that file3, since it's newer, and that it exists in dst/, it gets sent.
Testing
To make sure things work before you cut the command loose, I'd suggest using another of rsync's switches, --dry-run. Let's add another -v too so rsync's output is more verbose.
$ rsync -avvz --dry-run --update --existing src/ dst sending incremental file list delta-transmission disabled for local transfer or --whole-file file1 file2 is uptodate file3 is newer total: matches=0 hash_hits=0 false_alarms=0 data=0 sent 88 bytes received 21 bytes 218.00 bytes/sec total size is 0 speedup is 0.00 (DRY RUN)
