Existing files needs to be synced manually, there is a range select to help you with that however if folder is marked for auto-upload and upload constraints are met (only wifi, low battery, charger connected etc.) then any new photo you take shall be synced automatically.
There is also a feature which would help you to manage upload status if you switch to a new phone and connect to bucket previously used with PhotoSync (it would recognize file names and mark as uploaded accordingly).