We are migrating from Assembla.
Unfortunately I could not find a clean migration flow available for Assembla like some of the other agile tools. I found a github project for Assembla-Jira migration using API - but it is old and some parts of it are deprecated.
So, hopefully exporting out of Assembla and importing CSV would work. But I'll probably lose issue relationships like epic-story-subtask as well as 'related' links..
If anyone have had experience with it, please let me know.