If you have all your photos saved in the Cloud, then that is where the originals will be saved. What you can then do is view those photos on your iPhone, but in a smaller resolution, so they’re actually saved in the Cloud, not on your iPhone.
First of all, check that you know your passwords etc for your icloud account, so you won’t lose anything!
Then you need to:
- go into settings on your iPhone
- scroll down and click on Photos
- check that iCloud Photos is switched on (if it’s not switched on, when you do switch it on it will transfer all your photos from your iPhone to the Cloud, which could take some time if you have loads of photos)
- click on ‘optimise iPhone storage’
After you’ve done this, all the new photos that you take will automatically upload to the cloud, but will still appear on your phone.
Hope this helps!