Ogg Vorbis is a superior format for many reasons, not only being an open, royalty-free, patent-free standard. Just to name a few technical aspects, it has native metadata support, gapless play and multichannel, unlike MP3's ID3v1/v2 tags (v2 has no standard at all), player hacks to mitigate gaps (try listening to a live album) and 2 max channels (stereo). So MP3 is inferior in every way... its only "advantage" is being there several years first, got massively widespread, so any digital player (including car players, portable players, dvd players) can read it.

But, as for converting your MP3 to OGG... do not do this!

Transcoding to a lossy format will not improve its quality in any way... in fact, it may decrease quality! Both MP3 and Vorbis encoders are lossy, meaning they achieve high compression ratios by throwing away inaudible parts of the audio waveform. However, the MP3 and Vorbis codecs are very different, so they each will throw away different parts of the audio. The degraded quality may or may not be perceivable for a single conversion, but it will add up for each transcoding.

But if you have a lossless source (either FLAC, WAV, CD-Audio), and you want to convert from that to a smaller, lossy format, and you don't mind about portability (ie, you mostly in PC or in players that support it), go ahead and use ogg. It is superior than MP3.