According to this issue in the Atlassian backlog, it seems there may be some problems with trying to import to a Next Gen project from a CSV file. The last comment (currently) on the issue indicates a user was successful with the import except for the custom fields they created.
This reference document discusses the CSV import process.
I found another post from 2018 on this topic that seems to indicate import works the same for Classic and Next Gen projects, except for the problems with custom fields in Next Gen. (Google searches are a wonderful tool.)
I would say give it a try, but use a test Next Gen project to experiment first.
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