Read people read. The pic doesn't say anything about deleting the map. Just that you'll be forever locked out of Hardcore mode in it. You know until Single player commands counters this. Or your mod of choice.
I hope when you turn it to hardcore there will b a message saying 'Are you sure?' Then, if you say yes, it should say 'Are you really sure? I mean, if you die this worlds gone. Are you willing to risk it?' Then, after you say yes again, it will say, 'Alright, whatever you say...'