iCloud Safest way to change iCloud Photo library?

[Paco II]

I've got a 'newer' Photo library on my mac that is being used for iCloud Photos. I've got an 'older', much larger Photo Library that I want to switch to. The end goal would be to switch to the 'older' library, bring in all the photos from the 'newer' library, and then use iCloud photos with the new consolidated library. Step 1 is complete, in that I have copied all the photos from the 'newer' library into the 'older' library, such that the 'older' library now has everything. But I am less clear on how to proceed to Step 2, which is to switch to using this consolidated library for iCloud Photos. I don't want to create a mess. Would it be best to first clear out the current iCloud Photos entirely before setting the consolidated library as the one to be used for iCloud photos? And how would I do that? and what would happen with photos on my iPhone?

Appreciate the help. Really don't want to mess this up.

 

[Paco II]

Follow-up to my original post: If I remove the duplicate photos from the 'newer' library (there aren't many), and then set the 'older' library as my iCloud Photos library, will it merge the old and new photos from iCloud? Perhaps this isn't as complicated as I thought.

Appreciate the guidance!