Hadiths which has been already said as saheeh or daef can be made in to daef or saheeh, because those who collected Hadiths and made a judgment on it knew more of history than the books we got today regarding Hadith history, as many books have been lost which they knew about and wrote on. They are authority thus Muslims like Albani or Bin baaz and so on cannot make new judgment on Hadith as the did. To know about this see the book "Muqaddama Ibn Khaldun chapter on Hadith", Usool fi riwayah by Sheikh Mahmud abdul Khaliq" to name a few. Ibn Khaldun the greatest Islamic sociologist and historian of the Middle ages. Sheikh Mahmud Professor of Hadith studies in university of Al-Azhar, Hadith department.