Changing Apple Photos System folder

I have 120k of photos stored in Apple Photos in iCloud.

The HDD that I used to store the System Photo Library is randomly quitting and fails First Aid.

Time to evacuate.

I have created a basic Apple Photos library on my Mac, (today photos) and the copied it to a new HDD that is formatted MAC OS Ext Journaled.

I tried manually transferring all of the original photos from dying HDD by accessing the Photos/Package contents folder/Originals but the process fails randomly and force ejects the dying HDD.

So I presume that I can designate the new HDD with a new basic Photos Library as the System Photo Library and it will 'merge' the already uploaded 120k of photos on iCloud Photos and the new Photos Library?

And I can then manually download the originals from iCloud to my new HDD?

Thanks

John