Convert whole library to new .HEIC format Can the whole icloud library be converted from the non-optimised format to the new optimised format, or vise versa?

I have noticed that some of my photos have automatically be using the new format.
At present, September 2017, no. You will begin to take new photos in HEIC format once all of your iCloud synced devices in iOS and macOS have a version of the os and Photos app to support the new compression formats. 

Then, images will begin to collect in the new format. Presumably over time, it might get added either a until it’s to process or just a feature to go back and evaluate / reprocess the images if the potential savings are substantial. 

My hunch is that this would first be used with people using iCloud photo libraries and enabling the reduce file space option on some devices that then actually need space. The system will then be looking to offload / resample images anyhow and signal the willingness to have a transformation made to save space. Let’s see what heppens as this format rolls out this fall.