Diane would like to transfer all her Google photos up to another Google account. Leo says that now is the time to do it because Google is about to eliminate unlimited photo storage in favor of a 15GB limit. There is a third-party service called Cloud Transfer or MultCloud that can do it. But Google has TakeOut, which would allow her to download all of them and then upload all of them. Or, she could share the library to the new account and then copy or download them. That would avoid having to reupload.
From the chatroom - Cloudsfer, can transfer photos and data from one cloud service to another. And it has a free trial. That sounds like the way to do it.