Yea... I wrote a redmine to Jira importer. That was... interesting. I'm familiar with Redmine and its table structure. I was able to reverse engineer the json schemas (there are examples at https://support.atlassian.com/jira-cloud-administration/docs... but little in the way of actual schemas and DTOs to work from) and successfully imported everything including a number of custom fields in Redmine and comments and attachments.

The only "oops" part of the Great Import was that the users that were in Redmine but not in Jira (people who left the org and thus were never imported into Jira) had entries created as the sysadmin who ran the import.