Can I transfer a client’s book from my account to a new account in KDP Print?

I reached out to customer support at KDP Print, so here’s their official stance:

1. You can not transfer a title from one KDP Print account to another.
2. You can retire a title in one account and set it up in another account – this will create a new title on Amazon.
3. Technically, you can not use the same ISBN on the new title. If using a KDP Print-assigned ISBN, you definitely can not. If you use your own ISBN then the support rep I spoke with said he could make a one-time exception to allow the same ISBN to be used which would allow you to keep from creating a new page at Amazon and losing your reviews.

So…you technically can transfer a title if you use your own ISBN or don’t care if you lose your reviews by setting up a new page. Another option that you can use is to retire your title and create a second edition of the book in the new account. You can then contact support and have them link the two books so that people will see the new edition of the book and it will make more sense to readers on the site the difference between the two listings. They’ll still be two different titles (technically) but it will be obvious folks should get the new version.

Personally, if I thought it likely that I’d want to move the book, then I would just set up a new account under the other person’s name.

No matter what you decide to do, if you set up an Author Central account for this book, I’d set up a distinct one from your account under that person’s name.  You can create up to 3 pen names under one account, but I prefer to keep client books separate from my own where possible, and in Author Central there’s nothing wrong with having a new account under that person’s name so that it doesn’t co-mingle with your own books.