Migrating project form one RTC instance to another RTC instance.

wow, is it really 10 years later ?

Yes, miss those good times. I know you guys and others e.g. IBM internal, myself and others have coded up ways to export, massage and import data.

I still believe my assessment is correct.
There is no out of the box tool to migrate project area structure. You can create a process template and try that, you can code something using the Plain Java Client Libraries/EWM SDK. I have done partial implementations.
We have a new capability to export SCM Components with history. The distributed SCM allows bringing over the SCM data and history as well.
It is technically possible to recreate the history for work items using the Plain Java Client Libraries/EWM SDK. otherwise there are limited import/export capabilities for EWM Work items. You can use CSV and I implemented a special 2 pass implementation for the import that could keep a link back to the old work item - in the same repository. See https://github.com/jazz-community/work-item-command-line

Users have also used off the shelf 3rd party synchronizers to bring over data. Usually it is not lossless and usually the history is lost. The best way to handle the work item history, is a back link to the original work item. The new only has the history after the import, the old keeps the old history. Once proven there is nobody looking into the old system, switch it off.