As I take new photos I then airdrop my photos from iPhone to iPad and to two Macs and then copy them to an external for backup which can be a pain. I have my photos in folders by month and year - if I switched iCloud photos would the folder setup be retained?
Im sure years ago I tried it and just ended up with thousands of photos that ignored my curated system
If you have your photos by month and year, why not just use the Photo's App as this is how you can view your library?
If it is because you are concerned that your photos are hidden in some proprietary library, then this is not exactly true. You can easily can access to your original photos (unedited) by right clicking on your Photo's Library in finder and choose show package contents, from there navigate to your originals folder and then in the subfolders will be your original files.
A word of caution is don't move or edit your photos by digging directly in the Photo's library via the finder, but at least you know that if you ever needed to access the original files it is possible to copy them to another folder without the need to export them, in case the library becomes corrupted.
However to answer your specific question, in theory you could move your current photo folders into an iCloud folder, This would allow the folder to sync to any other device using the same iCloud account. However the Photo's App on iOS devices would not be able to see these files, and potentially you would be duplicating the storage of these files on your iOS device if that was the device on which where the photo was originally taken (it would be in the iOS photo''s library and in the iCloud library that was synced to your phone). For this reason I would not recommend it. In addition iCloud Photo's has specific functionality to manage the size of your photos library on iOS by only holding a small compressed file until you need to download the full resolution image for editing.