How do I get full resolution photos back onto my iPhone?

A week or so ago I turned on the optimise photo option on my iPhone but now realise that I actually need the hi res photos back on. Does simply turning back "download and keep originals" only work going forward or will it update the now lo res photos back to hi res? If not, what is he easiest way to get all my hi res pictures so I can put onto an external hard drive? I tried to do via iCloud but looks like it will take forever to do as you hav to select and save indivudually.