Transfer photos

Yes, once all photos have been synced from iCloud to your iPhone you can cancel the subscription for additional iCloud storage. You will have to delete the photos from iCloud Photos, before you can downgrade fro iCloud+. You iPhones most no longer sync with iCloud Photos, when you delete the photos from iCloud, or they will be deleted from the iPhones as well.

But wait, till all photos have been downloaded from iCloud Photos to our iPhones..

When you are syncing your iPhones with iCloud Photos, the photos will be in iCloud and shown in Photos on your iPhones. The Photos Library will be identical in iCloud and all your synced devices. Photos will sync any changes to iCloud and to all other synced devices. Any photo you delete from any of your iPhones will be deleted from iCloud and your other iPhone as well. Any photo you take on one of your iPhones will appear on your other device as well, any adjustments you apply, any Favorite Heart or caption will be synced across devices. So don to delete any photos from your older iPhone if you want to keep them on your new iPhone.

Your iPhones will download all high resolution originals to your devices, as long as you keep using the Setting > Your Name (AppleID) > iCloud Photos: Download originals.

By default this will be switched to "Optimize iPhone Storage" when you first enable iCloud Photos. Then the iPhone will try to free storage by replacing some high resolution originals by smaller, optimized versions for browsing and you need an internet connection to work with your photos. As long as you are having enough free storage on your new iPhone, you may want to use ""Download originals" and not "Optimize iPhone Storage".

log in on both with the same apple ID

then on both devices go to settings, your name, icloud

switch on Photos

then all photos are synced from one phone to iCloud and then to the other phone