Retain Unified Archiving 4.x
Created a new partition to have all new archives stored on this partition while leaving the existing data on the old partition.
Admin went into Configuration | Storage Manager | Advanced Settings | unchecked "Derive all file locations from above base path' and modified the 'Store attachments under this path' to point to the new partition in order to avoid increasing disk space usage on the old partition.
After the new partition was created, Retain will automatically save attachments to the new partition based upon the date that it was created.
Modified the 'Store attachments under this path' to point back to the original partition, then restarted Tomcat. Confirmed that all old and new messages could be read after that.