Hello Silcat, this is Emily.
If you remove the connection first, then delete the photos, you will be fine. The photos won't be deleted. Also, I do want to make sure you are referring to the Camera Roll upload right? If so, that's actually a backup, not a sync, meaning if you delete while OneDrive is still connected on the phone, your local photos on the phone are perfectly safe. You can read more about it here: https://support.microsoft.com/en-us/office/auto...