> 1) As Yahoo "upgraded" all password storage in UDB (where all login / registration details are stored) to be bcrypt before 2013, I'm curious how this was poss

> 1) As Yahoo "upgraded" all password storage in UDB (where all login / registration details are stored) to be bcrypt before 2013, I'm curious how this was possible. You check the plaintext password sent to the backend against the md5, on success you rehash it as bcrypt, insert it in the table.