once I start translating, is it possible to edit captions that have been already revised?
Or do I need to upload a copy of the video?
Once the captions are mark as completed, and you move forward with the translation, you can always go back and revisit the captions in case some changes are needed. From the video page, on the right panel, press the Video Properties drop down, and click on “Mark captions not complete”. That gives you the chance to re-make or correct the captions without losing any translation. Once you are done editing, mark the captions as complete, and continue with you translation.
Please let us know if there are any questions along the way.