Hello,
Good question, I opened a TMNT projects ‘.uefnproject’ file in notepad and am noticing the line "featureSet": "MeltedIronGFS"
I wonder if you remove this line, are you still able to see/place TMNT content?
I doubt it would change project type on https://create.fortnite.com/ though.
It should be possible to copy your existing content into a new non-tmnt template.