how do you merge iPhotos from an old MacBook Pro library into the photo library in the current icloud

If Photos will not open the old library and is giving an error message, that the old library has already been migrated, you may be having already a migrated Photos version of the old library. Look for it. If you no longer have the previously migrated Photos version or it is no longer up to date, you can open the old iPhoto Library or Aperture library again, by changing the filename extension of the library back to the original extension. remove the prefix "migrated" from the filename extension.

• For an iPhoto Library called "iPhoto Library.migratedphotolibrary" change the name back to "iPhoto Library.photolibrary"
• For an Aperture Library called "Aperture Library.migratedaplibrary" change the name back to "Aperture Library.aplibrary"

Is it urgent to merge the libraries now? The next system version macOS 12 Monterey promises that we will be able to import photos directly from one Photos Library into another library. It may be more convenient to merge your libraries after the upgrade to the next system und use the time until then to get the libraries shipshape.

See: https://www.apple.com/macos/monterey-preview/features/

Import photos from another Photos library

Now you can import original photos, including edits, from a second Photos library.

Hi.

Have you already converted the library for Photos by opening it in Photos?

If so, you can set that library to be the system library (photos preferences) and it will be merged with iCloud. Note - that merging will mean everything within iCloud will also be downloaded to that library.

You can then either decide to keep that library is your main library, or switch back to the original library as the system library, and let that sync with the new iCloud. You would then have two libraries with everything. You could delete the iPhoto one, or move it to external storage as an additional backup.