Continuing existing remote backups after replacing existing NAS

I've recently replaced a ReadyNAS Duo with a RN204. I'm now setting up the backups from this NAS to a remote ReadyNAS Duo. I'm used to the two NASs doing incremental Rsync backups where only the changed files are transferred. Because the files are primarily music and photos I've taken, files are generally only added and not updated or deleted. However, now I'm setting up the new backups, I'm no longer confident in what I'm doing. I see that I have an option to schedule full backups. With the old ReadyNAS Duo this is set to First Time but on the replacement RN204 this isn't an option and it seems that I have to have a full backup a minimum of every four weeks. I assumed this means that all data has to be sent again which is not desirable as there is over a TB to be exchanged but I'm not an expert in Rsync so I'm now not sure.

Is Rsync the right thing to be using here and is my understanding of what a full backup represents correct? I'm keen not to have to send TBs across the internet on a regular basis!

Thanks!