Since the association between the user and the book is in the license, the only way to get the book associated with a different user is to get a new license. This is not something that Adobe can do, so your best course of action is to issue the customer new download links, so that they can get the content associated with their new AdobeID.
I have done this as well as when I bought my sony 600 reader it would not allow ebook library onto my computer a sony vaiou running vista premium service pack 2 in fact it still will not load.
so my husband downloaded adobe digital onto his mac got ebook library working --bought me 2 books from waterstones and then the fun started --we got them onto the reader but they are protected and no matter which machine we go on it will not let us see the books
HELP HELP HELP ready to take this back and get a Kindle
Today I have spent 10 hours on this and no support from Sony or Adobe or Waterstones the whole thing is a nightmare --worse is I am not alone in this
You are certainly not alone, Chrissie. I have been caught out by the same thing. I will be advising everybody not to buy an eReader until this problem is sorted out, and would hope that everyone who reads this post will do likewise.