set primary backup chain

Is there a way to set the primary chain?

Use case: backing up over a slow internet connection, and wanting to keep using the existing full backup for increments while a new full is uploading into a temporary directory. When that new full is finished, move all the files into the main backup directory. duplicity detects the new files and rebuilds the local cache, but then increments are still against the original chain. is there a way around this?

I suppose I could alternate between the two directories and not move the files, just have two directories