An alternative to Mac Jim ID's method is to share the photos. Read this support article: How to use Shared Albums in Photos on your iPhone, iPad, or Apple Vision Pro - Apple Support
Note that if you use Mac Jim ID's method you will want to make sure you are not using optimized photos. Here's a section from my account merging post that relates to that:
If you have signed into the first Apple account on a mobile device (but maybe also on a computer?) then fortunately when you sign out of that account it asks you if you wish to retain the data from the old account on your device (toggle the ones you want to on What happens after you choose Sign Out But Don't Erase - Apple Support ). If you keep data on the device then when you sign in with the other account it will merge this data with the account signing into the device. You can later delete individual items from the old account that you do not want. Similar information can be found in this post: I have 2 apple IDs i want to merge them h… - Apple Community , and a step-by-step in this User Tip: How to “unshare” an Apple ID - Apple Community . Note that only content physically present on the device such as contacts, photos and videos will be retained. Meta data such as a game play scores that are tied to a specific account or are only on a vendor's Apple Account-specific database remotely will not be retained. Also turn off storage optimization before doing this. With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. Make sure photo optimization is turned off so the full quality versions from the old account are actually on the device before merging the content into your new account. Manage your photo and video storage - Apple Support You can turn optimization back on again once you are fully using your new account on the device.
User level: Level 1 (4 points)
