I've searched a lot when I was contemplating creating a new Apple ID on how I could transfer all my data from one account to another. I have around 35,000 photos on my original iCloud account and found a neat way to transfer them all, as is, to a new iCloud account. First, you will need to be logged into your Mac with your original iCloud account. In the Photos.app, you should have your full library downloaded to this Mac (not optimised Mac storage option). If it isn't fully downloaded, then wait until it is before proceeding. This will leave you with a complete local backup of your photos on your hard drive, usually stored in your $User$>Pictures folder. Close Now sign out of your iCloud account under Settings > iCloud (bottom left option). Keep in mind that by turning off iCloud will affect other iCloud services such as iCloud Drive, notes, reminders etc, so make sure those are backed up/transferred before you do this. I'm just focussing on photos for now, but if anyone needs help with those other features then let me know. Now sign into your new iCloud account (the one where you want the photos to be transferred to). Open up Photo.app. You should see all your photos and albums as you left it. If not, then close Photos and [Option]+click the Photos.app icon to select the library if you have it saved elsewhere. Now for the lengthy part - uploading all your photos to the new iCloud account. Once that's done (many many years later), you should be left with the exact same photo library as your previous account. Done! ----- The alternatives I almost tried (and glad I didn't) were to export all the photos and videos and then import them to the new account. I tried a few photos but found that not all the meta data was being preserved and you only get the option to keep edited files or their originals, without the option to roll-back. This would have been a NIGHTMARE to manage for 30,000+ photos! The other alternative was to buy a license for PowerPhotos and have it copy the library to another account. Same drawbacks in terms of only being able to store edited files OR originals, and burst photos were not preserved amongst other things. Not to mention the license for the full version was coming up to $30 or so. End of the day, I think my method above is the best despite it taking days/weeks/months to complete and hope that it helps someone else. With Apple's restrictions on changing apple ID's, merging ID's, you needn't worry about transferring photos from one account to another. Any questions, please ask!