I'm curios if I can convert an existing Next-Gen project with issues and settings into a Classic project? Say, I'm unhappy with the Next-Gen feature set and would like to access the features that are only available to Classic projects, which I'm used to. Obviously, I would make a trade-off and lose the features that are only available on Next-Gen, but ultimately that's a trade-off I might be willing to do in certain circumstances.
If that's not possible, is it possible to export issues that exist in a Next-Gen project, create a Classic project and then import them there?
You probably notice what I would like to achieve, so if there's any other way to get existing issues and some settings, workflows, etc. from Next-Gen to Classic, I would be more comfortable with starting a new project with Next-Gen if I would know I can convert it to a Classic one if I really want to access some features I might end up missing from my experience with Classic.
So, after some research, I found out, that the most convenient way to do this is to:
I guess this is the way to go if one desires to move from next-gen to classic, since the description under "Back up for server" says "Bulk move important issues from next-gen projects into classic projects administered by schemes. Then, delete your next-gen projects." So, even though it's thought as a way to move to non-cloud projects, it can be used in this way as well.
I didn't go through with the CSV export-import madness, so I explored other options and stumbled upon the backup manager (which isn't the first option I would go for related to moving issues from existing project to another). As this wasn't explained in your answer, I'm going to expand on it a bit in another answer.
I noticed on the "Related Community content" immediately after posting a quite similar question, but the other way around: https://community.atlassian.com/t5/Jira-Software-questions/Migrate-a-classic-project-to-a-next-gen-project/qaq-p/920382