It worked, thanks. I deleted all lines (exported them as csv first not to lose data) and when the form was filled for the first time after the deletion, the columns were populated.
I have another question though, that came up when exporting the data: is it possible to bulk export attachments (my contact form has a “send file” option)?
By the way I just noticed that after I export a cvs file, there are two columns called “Position”, so I can see it is using the first one to display by CFDB7. Do I have to uninstall and reinstall the plug-in if I change the format of the form?