Bear in mind that rsync normally uses timestamp and file-size information to determine if a file has been "touched," although it can also use (slower ...)
However, your basic premise is a sound one. If the amount of data is very large, "sneaker-netting"
the first set of data to your office might be expedient.
sure that the backup (and all
other dealings with your remote office) take place using certificate
OpenVPN (or IPSec) connections. Although --rsh=ssh can
provide cryptographic protection for the transfer, exposing ssh
to the general public invites "kiddies" to start pounding at your door.