If you saved/archived each new json formatted feed list reply as yyyymmdd_feedlist.json (for example), when ever looking at/for downloaded files by feedid, it should be easy to marry up to a feedlist by checking the downloaded feed file’s creation date properties, or date of last change if you are not editing the downloaded files.
It’s certainly not the only route to take. There is the emonPi/SD backup module for emoncms (I have never used that), there is also the emoncms sync module (I tried that briefly in its earlier days) and there is also backup/replication in the the emoncms/usefulscripts (I think this is what you have now found?)
You can also make the api calls directly from a script of your own to create your own custom routine, I have previously used a combination of my own custom scripts and the backup/replication script to achieve what I need at the time.