Ubuntu – Convert mp3 to ogg

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.

Instead of being "a bit scared" or following unrelated links, why not simply read the manual?

From man mp3gain:

   mp3gain optionally writes gain adjustments directly into the encoded data.  In  this
   case,  the  adjustment works with all mp3 players, i.e. no support for a special tag
   is required.  This mode is activated by any of the options -r, -a, -g, or -l.

   If none of the above options are given, the recommended gain change is instead writ‐
   ten  to  a special tag in the mp3 file. In this case, the adjustment only works with
   mp3 players that support this tag.  Some mp3 players refer to  this  as  ReplayGain.
   The  tag is written either in APEv2 format (default) or in ID3v2 format (with -s i).
   If you only want to print the recommended gain change (and not modify  the  file  at
   all) you may use the -s s (skip tag) option.

So it only modifies the audio stream if you tell it to, otherwise it writes to tags only.

Side note: many of the URLs posted in comments are not related to current mp3gain found in Ubuntu: they are either terribly outdated or they refer to the windows version.

Whenever in doubt as how a given command works, always try man first. If that is not enough (or no man page available), use the correct upstream website, which can be checked in Ubuntu's package page.