I’ve rebased my hack to change the repository id or to recreate a lost config file (some parameters are still lost, but reading should be possible) GitHub - MichaelEischer/restic at change-repo-id .
To use that branch one has to build it using go build -tags debug ./cmd/restic and then run restic debug changeID i-understand-that-this-could-break-my-repository-and-i-have-created-a-backup-of-the-config-file placeholderID and finally replace the placeholderID with the ID printed in the resulting error message. That code is pretty experimental, but as it at most modifies the config file, it should be enough to just create a backup copy of that file.