So, for the first question, no, it is not possible to use the API to alter ids like the author of a comment or an Epic. Just imagine some malicious actor wanted to create false testimony of someone, that would be troublesome.
As for meddling with the history keys, that might break stuff, yeah, particularly with ids. But that in particular, only getting the history refs might work, but it may then break because of the epic belonging to another project.
To be fair, we have not done those tests in the past, so if you feel like trying, please do let us know the results. Else I’ll try myself, but can’t give a timeline since we are quite busy at the moment.
In the end, I solved it a slightly different way. Remembering how I made my migration work from Pivotal Tracker to Taiga, I wrote a ‘manage.py’ command that directly interacts with the database (not via the REST API). In this way, I can preserve the comment authorship.
Here’s how I did it (might be useful for others who come across this thread):