It will definitely create a problem for that user as thier system, would consider the second account an alt of the first one and eventually itll be banned. In the meantime if that user was to deposit funds and win, then too he would not get paid by FJ for breaking their policies. Im not sure if one can ask the support staff to delete his old account and allow him to use their new account on FJ, you can inquire this with their support staff.
It will result in lots of drama that site's owners don't want to see for sure. It is same things as we have in the forum. Someone can quit, stop using their accounts in the forum, delete all of their past posts, except threads; or moderators can nuke those accounts, but accounts are unable to delete in the forum. There are some limited accounts were changed usernames, but nowadays, such things hardly to accept by admins. All of them for site's security and preventive methods to out of drama.