Rather that directly answer your question I think I'll just point out that this is probably a bad idea.
If you are converting between different codecs, this will involve not only a deterioration in quality but also potentially wasted disk space, since each codec has its own characteristics at a given bitrate. That is, it's possible to transcode from format A to B and have a larger file size but reduced visual quality.
I'd advise keeping the videos in their original format, and only converting them when absolutely necessary, such as:
When the videos are at a very high bitrate and you cannot afford the disk space; you want to use a significantly lower bitrate.
When the videos are in a format that is not supported by computers or devices you own or are in a "dead" format nobody is likely to use anymore.
When you are outputting to a specific target, such as authoring a DVD disc.
In all other cases, why not retain videos in their original format?