I have used duplicati for many years and very happy with it. My backups keep multiple versions of my files for up to two years. I believe that major duplicati revisions have changed the local sqlite database format. Are the backends (dindex and dlist) formats ever changed? I ask only if there are occasions when I should delete the complete backup and start over with a fresh backup.
Hi @gscott, welcome to the forum 
Generally no. For the duration of the project there has only been one file format version.
All the remote files contain a small manifest file that, among other details, contains the file format version, and the version of Duplicati that created the file.
If we need to alter the file format in a way that is not backwards compatible, we can do it by increasing the file format version, and this way, older versions would refuse to read the files.
In the very early days of Duplicati, we did extend the index files without increasing the file version, because the older clients could still handle it. Other changes was the introduction (and later removal) of 7z compression.