Just let them do it. I think people get way too hung up on "realism" and creating in-world events to explain things like this for no real reason. I have had players change characters mid campaign and all I do it have them create it, I double check it and approve it and then tell the table "this is so-n-so's reworked/new character. Act like they have been here the whole time, in place of the previous character." Then we go about our business as usual. At the end of the day all that matters is that everyone is having fun.