iPhotos Library to Photos migration

Hi there, I'm a bit stuck with something - I had an iPhone 5 from 2013-2017 and a MacBook Air (2013) and I backed the photos taken on my phone manually by importing them to iPhotos on the laptop (I didn't like the iCloud photo stream back then). The iPhotos library on my laptop was taking up too much space, so I moved it onto an external hard drive. I now use an iPhone 11 and new Macbook, and since 2017 my photos are all stored on iCloud. I want to access the old iPhotos library with all my photos on from 2013-2017, but I get the below error message when I try to open it "The contents of the iPhoto Library "iPhoto Library" have already been migrated to Photos. Open Photos to view your library or use a compatible version of iPhoto to access this iPhoto library."

I have followed the steps elsewhere on the forum about copying the iPhotos library to my laptop's Pictures folder, and then holding the option key down while opening it, but then this message comes up "Switch Libraries Do you want to switch the current Photos Library from "Photos Library" to "iPhoto Library"?"

- I have clicked cancel so far, because what I DON'T want to happen is it to replace my current photo library (i.e my iCloud library with all my photos since 2017) with whatever is on the iPhotos library from 2013-2017.

Is anyone able to explain what will happen if I click "OK", and whether this message is saying it will just transfer the old photo library into a compatible one, rather than replace my current one with the old one? I don't want to lose any photos, especially not anything from the last 5 years!

Thank you!