If I switch to download and keep originals on my newer iPhone with plenty of storage, will the full versions of all past photos be downloaded back to the newer iPhone?

Hi there, Tahitipete.

I see that you're using another iPhone with more storage available. You'd like to know if you stop optimizing the storage for your photos if the original versions will download. Yes, they will.

Manage your photo and video storage -- Once you turn off Optimize iPhone Storage under Settings > [your name] > iCloud > Photos, your iPhone will start replacing the smaller, space-saving versions with their original, high-resolution versions from iCloud.

Take care.