If you're new to next-gen Project, you might have noticed how easy and elegant it appears when using it. Likewise, it has an easy to use Permission which makes it a definite template to use for small teams or even projects to get set up fast.
However, let's say you're actually moving a previous project from Classic project into next-gen project, there are certain things to take note of as mentioned on this article and one of those things would be having your Epic links removed or delinked from the issues.
Epic links: Links between epics and other issues (Story, Bugs, Tasks, etc...) in your classic project won't exist in your next-gen project. The issues themselves will still exist, but the links between them won't.
That's a real pain; what If I told you that there is a way to actually get back all your Epic links after the bulk move in your next-gen Project?! Well, it's no secret but using Jira's REST API this is easily achievable on your next-gen project.
All you just need to know is to be able to run a script on a terminal that will help in performing the bulk action and we just have the right script, which can be downloaded from this Github repo here
Instructions on how to use the script is mentioned on the README.md file on the Repo. Just save the file with a text editor in a .py extension and download the required "requests" module as its written in python. Startup the script and follow the prompt to correct the Epic Links in your next-gen Projects.
Feel free to ask any questions about using the script.