I took a different approach. I reset the kindle and left it, just downloaded collections and no books. It slowly has started to fill the covers in. Much slower than my 5th generation, which populated the homescreen in seconds
If you remove a Fire from your account then it's contents is automatically removed.
If you remove a Kindle from your account then the contents remain on it - generally until you register it to a new account or do a full Factory Restore (pretty sure the Factory wipes it, but not certain.)
So yes, the "old" kindle will have repopulated in seconds. Indeed, it never de-populated.
The good news, I hope, is that you are now confident of the Cloud Collection system working and therefore can decide how small a "library" of books you want to walk around with on any device.