The problem is that the internal storage is encrypted, so the only hope would be to somehow move the images to Android secure folder which will not be affected when you reformat the SD as External.
Easy to say but I’ve not had the problem and don’t know how to do it or where to transfer the files to. Hopefully someone else will have more detailed ideas.
Is your phone unlocked or rooted?
The other option is to upload your images and so you will have a backup elsewhere before you reformat the SD card.
For other reasons not to use as internal storage see