So I have finally solved this by exporting the database backup, then editing the .db file with an editor and replacing every mention of freenas-boot with xxxxnas-boot (exact name probably does not matter except it seems to be inportant that the number of characters it the same). Then saving the file, putting it back into the .tar with 7zip and then importing the databse backup. Then I was able to export the xxxxnas-boot in GUI.