This is one of the reasons the big boys render to uncompressed frames. So long as you keep those raw frames and do your compression in a video editor, you can recreate that video in any format you choose. A lot of codecs are free, a lot cost a small bit, some cost $. But the codec is installed in your OS, so you can have, say, DivX, Xvid, Quciktime Pro, and one of the MP2 and Mp4 codecs, and you can create video in each format. Doing so from umcompressed footage avoids the kinds of artifacting you tend to get when you take one video format and use a converter program (pixellation and mosaic patterning being the two most common). It also allows you to take the raw frames into an editing program like Photoshop or Gimp or PSP and apply effects or corrections. It really opens the flexibility.