Changing/updating system photo library

Hi

Yes, that is the correct approach, assuming:

If the common photos (those in both libraries) have been imported from the same original files, then duplicates will not occour. However (for example) you have exported edited files from one of the libraries, and re-imported those into the other, then as far as photos are concerned they are different versions and both will appear in your merged libarary. Same if one library contains original files and the other contains versions edited in any other software.

You should leave photos open in the background to sync - and by changing the system photos library, you have triggered a complete resync of the whole library. This will likely take days (library sync is normally much much slower than your internet speed would allow). For my 50GB library it took a little more than 48 hours. You should probably leave your Mac on with photos open, connected to power for 24/7. If you look at the bottom of the all photos view it will give you a (sort of) progress indication. But this will often not change for a long time.

That is the normal speed. When I make a complete new upload of my library with 55000 photos and videos, the upload will roughly take ten days. So, you should also expect the upload to take several days for your library. It is not just a data transmission - Photos has to merge the new system Photos Library into the existing library and resolve conflicts between duplicates. This syncing and checking for duplicates will be much slower for a large library, as each item from the new library has to be compared to every item already in iCloud, and Photos has to solve conflicts, if duplicates from both libraries are having different metadata or are in different albums.