rsync. It will synchronize the directories in one direction. So, if you want to update your
old folder with everything from
new, but keep what's in there, just use:
rsync -avh --dry-run /path/to/new/ /path/to/old/
This will, in a first instance, just output the list of files that would be transferred. In that case: Everything found in
new will be copied to
old, unless it's already there. Everything in
old stays as it is.
If it looks fine to you, remove the
--dry-run argument to transmit them for real.
-avh flags just enable archive mode (which will preserve timestamps, etc.), verbosity and human-readable file-sizes. Nothing will be deleted from the destination unless you specify the
--delete flag. Consult
man rsync for more information.