Thanks for that info and choosing the Apple Support Communities. It sounds like you want to take photos associated with your old Apple ID and have them on your iMac. To do this, please follow the steps below, and then you would be able to associate them with your new Apple ID.
You might already have a copy of your library in iCloud Photo Library, or the last 30 days of photos in My Photo Stream*
If you enabled iCloud Photo Library, your photos and videos are automatically uploaded to iCloud at full resolution. You can make copies of your photos and videos when you follow these steps:
- On your iPhone, iPad, or iPod touch with iOS 10.3 or later, tap Settings > [your name] > iCloud > Photos. If you’re using iOS 10.2 or earlier, tap Settings > iCloud > Photos. Then select Download and Keep Originals and import the photos to your computer.
- On your Mac with OS X Yosemite 10.10.3 or later, open the Photos app. Choose Photos > File > Export.
- On your PC, set up iCloud for Windows and turn on iCloud Photo Library.
If you want to download a few photos or videos, you can use iCloud.com to download the most recent version of photos and videos to your computer. Just go to iCloud.com and open the Photos app. Then select the photos and videos you want to download and click .
My Photo Stream* photos are automatically imported to your Photos library on your Mac, unless My Photo Stream and Automatic Import are turned off in the app preferences. If you turn on iCloud Photo Library, you can find your photos in the All Photos folder instead of My Photo Stream.
You can also use the steps below to manually copy your photos and videos. Any comments or likes connected to a shared photo aren't saved, and photos and videos saved from shared albums aren't full resolution.
Archive or make copies of the information you store in iCloud
Follow these steps to manually import photos and videos in shared streams to your Photos Library:
- Open Photos and select the Shared tab.
- Double-click a shared stream.
- Right-click (or Control-click) a photo and choose Import.
After you import your photos, you might want to back up your library. You can store them as a separate library on your computer or on a different drive.