Nathan Gray made this suggestion, following it up with a UI example

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Another thing I am interested in having is the ability, when
converting from a darcs-1 repo to a darcs-2 repo, to specify
already converted darcs-2 repos as a reference for the
conversion.

We have about 15 copies of one of our repos, and rarely are there
more than 5 or so that are completely in sync.  To convert from
darcs-1 to darcs-2 will be almost impossible, unless I can
convert each one on its own.  Currently, this is impossible,
because the conversion is lossy, and may make different decisions
based on which patches are in the repo.

I would like to be able to convert the repo with the most
patches, then for each additional repo, specify the already
converted repo(s).  If a lossy decision is to be made, the
reference repos can be consulted and make the same decisions.

It might also be satisfactory to convert multiple copies of a
repo at the same time, though if I forget about one, I will still
want to convert it later.

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darcs convert source_darcs1_repo new_darcs2_repo --use-reference
already_converted_repo

Seems like a duplicate