A good use of frame by frame will always be appreciated on newgrounds.
"tweening" is also a great way of filling in time. But, it can easily be exercised in a tacky way. I use tweening for landscapes, or introducing a setting.
for things that you dont want to draw all over again... you can draw it all once. then you can copy that frame and use it as a template, erasing what you can and drawing it over again. either way this is going to take forever.
If your looking for a time saver, you are still going to spend many hours.