Definitely back up your video before you try converting anything. I'm not sure about online tools since I haven't used them, but you could give this converter a try. Another option is to use VLC media player which is supposed to be good for making conversions. Just be sure to choose a lossless format as your output.
Someone once mentioned that VLC is a good converter but I have never thought of using it before. I try to avoid the free online tools but there is every possibility that there could be a number of them that stand out!