Database migration failure when downgrading 6.4 .0 -> 6.3.0

Hey all, was hoping someone might be able to help, I’m attempting to rollback my concourse DB from 6.4.0(1591737494) -> 6.3(1561558376) however, when running the migrate command I get:

# ./bin/concourse migrate --migrate-db-to-version=1561558376
error: Could not migrate to version: 1561558376 Reason: 1 error occurred:
        * Migration '1579713176_version_md5_resource_cache.down.sql' failed: 2 errors occurred:
        * Transaction CREATE UNIQUE INDEX resource_caches_resource_config_id_version_params_hash_uniq
  ON resource_caches (resource_config_id, md5(version::text), params_hash) failed, rolled back the migration
        * pq: could not create unique index "resource_caches_resource_config_id_version_params_hash_uniq"

Has anyone seen this happen before and know what can be done for us to perform the downgrade?

Thanks,

Ash