I'm looking for the best way to migrate multiple projects and the issues, history, comments, etc for each project to another instance of JIRA. The instance I'm migrating from is very messy. The majority of projects contain custom workflows, fields, and plugins, so the configuration needing to be in place to import with project importer would be a huge pain. I'm hoping what might work is the method of taking an entire JIRA backup of the instance we need to migrate, restore the instance, delete the projects that do not need to be migrated, take a backup, and then import that backup into the existing JIRA instance that we need to migrate the projects into. If I understand this method correctly, this should bring across the projects, project configuration, tickets, comments, and basically all of the history for the projects we're migrating.
Now, my big question is... is this method even possible when importing into an existing JIRA instance? The documentation I read recommends this method if you are importing into a new empty instance. We're not importing into a new empty instance, we need to import the projects + all history from Instance A into a pre-existing instace of JIRA B, without losing anything from pre-existing JIRA instance B. What is the best way to do this?