At this moment, after an admin (or user) creates a Next Gen (former Agility) project, it is not possible to change its type to a "regular", non Next Gen one (or vice-versa).
Add the ability for admins to change the project type of an already created Next Gen project (or vice-versa).
Next Gen projects are a nice way for a team which is not used with Jira to get work started without complexity. Yet, once the project matures, the Next Gen type may not be as flexible as regular projects (at least not at this moment).
Create a new "regular" project then bulk move the issues from the Next Gen project to the new one.