@odd rover You need to modify the "Version" part of your save file, as this handles the game mode type of that save. Save Editor > Edit Raw JSON, and you'll have to change the number to the "version" you want the save to be. It should be right at the top of the raw JSON of the save file.
The number each gamemode is associated with changes regularly (it seems to increase incrementally with updates), so the best way to do this safely is to make a fresh save in the game mode you want to change your current savegame to, and open it up in the save editor to get its current version number. Then you just paste it into your main save and that should work, as I used this method a couple years ago and it worked fine.
As an example, my Normal Mode save is currently Version 4638, and if I wanted to change it to Permadeath, I'd make it 6686, as that's the current Version number I've got from my Permadeath save. I can quickly grab the version numbers for all four modes if you need that.
I hope this helps.