Here are a few methods to transfer photos and videos from one Google Photos account to another Google Photos. When using these methods you transfer edited copies, without the option to undo the edits in the receiving account.
Warning: photos backed up before June 2021 to the old account with setting "High quality" (now called "Storage saver") do not use any storage on the old account, but will use storage on the new one.
Each of the methods below has advantages/disadvantages:
Method A looses Descriptions, Locations, Date/time added or changed in Google Photos.
Methods A and B are more laborious than method C
Method C is the best one to transfer all photos, but does not transfer albums.
You can use method C first, and transfer albums later using method B. By doing so the photos should not transfer twice, but will be added to the albums.
Also remember that having only one copy of your photos is always unsafe. If you do not have a real backup I recommend to for example also download all photos or have them in two accounts.
Some useful additions to this article:
https://support.google.com/photos/thread/240328793/transfer-all-photos-to-a-new-gdrive
If you no longer have the original photos and your photo collection exists only online at Google Photos, you'll need to first download all your photos and videos from Google Photos to a computer.
At this link are various ways to download photos and videos. Takeout downloads original and edited copies. All other methods transfer only edited copies
At this link are various ways to upload to Google Photos.
What is transferred?
"Descriptions", date/time or locations added or modified in Google Photos, get lost when downloading.
Takeout downloads these data in .json files but you will have to add them to the photos. See link.
Albums can be transferred when you download and upload per album. Any additional information added to the album (text, maps, locations) is not transferred. You will have to add or copy it manually.
Share an album
Open the album in account 1 and
check in 3-dots > Options whether "Share location" is turned ON/OFF
copy the link: Click the "Share" icon > "Get link" or "Copy" when already created.
Go to a browser window where you are signed in to account 2, and open the album
Select all photos in the album (or the ones you want to transfer).
You can now:
Add the photos to an album in account 2:
Click the + icon ("Add to"), and select "Album" > "New album". Add an album title.
The photos are now added to the album, and automatically also added to the library of account 2.
If the photos were already saved to account 2 (for example using the "Share library" method) they will not be added again to the timeline, but are still added to the album.
or only add the photos to the Photos tab by clicking on the "Save photos" icon (cloud with down arrow)
Share selected photos by "Send in Google Photos"
You can select photos and click Share > "Send in Google photos". You will find the photos in the "Sharing" tab in the other account, where you can "Save" them to Google Photos, but not add them directly to an album.
What is transferred?
"Descriptions" or date/time added or modified in Google Photos are also transferred.
Estimated locations or locations added in Google Photos can be shared or not.
See https://support.google.com/photos/answer/6153599
Any additional information added to the album (text, maps, locations) is not transferred. You will have to add or copy it manually.
"Comments" and "Likes" can also not be transferred.
Warnings
Please have a backup of your entire collection in case anything should go wrong while sharing. You can first use method C (Share library) which already transfers all photos/videos, and restore albums later using method B.
The photos will be added to an album, but not transferred again (no duplicates, but it is good to check).
Albums shared with you will have to be transferred manually. Open the link in the new account and "Join" the album.
There have been reports of reduced pixel count when using "Add to library." It seems that the smaller copies are replaced by larger copies in the shared album after some time. Check the pixel count after adding to library. If the pixel count is too low, wait a few minutes and check again.
In June 2017 the option to share your entire library with a single other account was introduced. This method can be used to transfer photos between two accounts:
In account 1: share your library with account 2
In account 2: accept the invitation and activate "Turn on auto save" .
Now photos from account 1 will be copied to account 2.
Deleting photos from account 1 will NOT delete them from account 2 after they are saved to account 2.
Give it enough time to copy everything. It is also wise to have another backup (non-synced copy) before you start deleting from account 1.
From the Google Photos Help page, please read: Share your Google Photos library with a partner
What is transferred?
New Nov. 2024: Pictures you’ve downloaded from other apps won’t automatically show up on your partner’s phone. See Google Photos Partner Sharing is less of a snitch now
"Descriptions" added in Google Photos or "Captions" added in Picasa Web Albums are transferred. Note that these get lost when downloading (method A)
When a date/time was changed in Google Photos, the changed date is also transferred.
Photos that are moved to the "Archive" in account A are not shared, thus not transferred.
When a photo was edited, only the edited copy is transferred. Edits cannot be undone in account 2.
The above method transfers photos, but no albums. You can however:
First share the library of account 1 with account 2. That's rapid and fully automatic, and includes photos that may not be in any album.
Transfer the albums later by using method B. Photos will not be transferred again but added to the album. There should be no duplicates.
The help says: "Any photos you save from partner sharing won’t take up storage, unless your partner stops sharing their photos with you, or deletes the original copy" . However there is currently a bug which prevents the storage being used in your account if the source account is disconnected. This may change any time in the future so if you suddenly see your storage jump it could mean they have fixed it.
When the option "Storage saver" is selected in account 2, the photos will be compressed and use less storage in case account 1 used setting "Original".
Can transfer between Google accounts, but also to other photo sites. Users suggested for example
https://www.multcloud.com/
See https://www.multcloud.com/tutorials/transfer-google-photos-to-another-account-1234.html and
https://chromewebstore.google.com/detail/transfer-google-photos-to/lihpabnkbapgdlgcibhnibeomajlgmgm
Reviews: https://www.google.com/search?q=multcloud+review
https://www.picbackman.com/
Comment by a user: "Picbackman can't transfer from one google photos account to another. Only to a different provider."
Method C (Partner sharing + saving) seems the easiest way to rapidly transfer all photos/videos to another Google account. If you want to transfer albums, you can do that by using method B, also after having used method C.
Since albums show the number of photos in the album, this method allows to compare the numbers between the old and new album, and thus check completeness. You can do that on existing albums, but also on albums you create specially to check completeness. Albums may contain up to 20.000 items.