That would fix part of the problem, and I'm pretty good at converting images from one format to another -- plenty of tools and experience. However, my previous books in Aperture were formatted as softcover, 8.5 x 11 inches (convenient for a casual coffee-table book, which is what these were). That option doesn't exist in Photos: I can get hardcover at that size, or softcover at 8 x 8 which requires me to reformat the book. With that realization moving the photo book to Photos isn't really an option for me: I might as well start over and create a new book in Photos, and just import the images. The alternative (which I increasingly prefer) is to export the book as a PDF and find a service (e.g., Presto) that will print the book from the PDF without a lot of fuss.
I must say that I am not keen on the current version of Photos to create photo books -- pretty restricted layouts. Perhaps that will improve with time.
I'm also not keen on some of the services out there: try to get Shutterfly to just take your PDF and print it. Just try. Ditto MyPublisher. They all want you to use their on-line page layout system. Hey guys, I've done the work already.
I also confess I don't understand why Apple turned off their printing service within Aperture -- the people who still use Aperture, like me, have already made it clear that we prefer that to Photos or Adobe Lightroom, at least for know. Deprecating a very useful feature isn't going to get me to move to Photos, it's going to make me find a work-around.