Recover indice(s) when changing gateway from S3 to Local

I have changed the configuration of an ElasticSearch instance to no longer
us the S3 Gateway plugin.

I have all the pre-existing indices on the filesystem, but they are not
recognised by ElasticSearch on re-start.

Is there anything I can do to "recover" or "re-index" these indices?

From what I understand, it is because they are missing their corresponding
"_state" directories and meta.

Many thanks in advance for any help.