I would imagine the "Adopted Big" system is valid. I mean, I don't think Bigs and Littles are registered with Nationals or anything, so it would just be a personal choice, right?
About your big, it would be really weird to have your old big listed on family trees, b/c it's not like she went Undergrad Alum...she deactivated. Sorry I couldn't be of more help, I'm a PC '01 so I don't have a little of my own, either, and haven't been through this yet.
I'm sorry your big left...it must hurt. My grandbig went Undergrad Alum last semester and I could tell my Big was really saddened by it.
