This post assumes you already have backups of your rrds
Lets say, as happens with #9903, that a sensor index changes for whatever reason.
This will make your LibreNMS install to delete the old sensor (and historical data) and create a new one.
The only you need to do to keep the historical data is rename the old rrd file to the new one.
You will probably have a gap in your graph, how big will depend on how late you rename the old rrd from backup to the new one since the update happen.
Hope this little guide helps.