Way more thorough and up-to-date information is available at Montgomery GI Bill Refunds.
I've seen some questions about service members who bought into the Montgomery GI bill and are now eligible for the new Post 9/11 GI bill. The question is answered at the VA website but it isn't written very clearly. As it is written, it seems to say that if you bought into the Montgomery Bill, and you choose to use the Post 9/11 Bill, and you use all of it, you can get a refund of the $1200 Montgomery Bill buy-in.
Please, if you know more, or if I am wrong, let me know in the comments.