Unfortunately, going the other way around is a difficult process, which is really, really ironic. If you don’t do this, photos/videos that you upload to Google Photos will be the “optimized” version, which means it will look like crap.
When asked, select to download all photos/videos back to your device in original quality. This is really easy if you still have your iDevice around. Here are some of the things I learned while using Google/iCloud Photos: Moving from iCloud Photos to Google Photos
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