Yeah you really wouldn't want to convert anything but WAV or any other lossless format into FLAC (mp3 and Ogg are lossy formats). Converting a lossy format (mp3 or Ogg) to a lossless format (FLAC or WAV) wouldn't recover any of the lost data and would be rather pointless. Converting WAV to FLAC, however, will reduce the original filesize to about 50-60% of its original size, while still maintaining the exact same audio quality as WAV. Converting FLAC to mp3 or Ogg will reduce the filesize even more but will remove some of the actual audio data (higher bitrates retain more of the original data but still remove some, though typically out of the range of what you'd notice listening to the song). At bitrates like 96kbps or even 128kbps, you'd probably notice a difference in audio quality, whereas with a 256kbps or 320kbps mp3 you probably wouldn't be able to tell much difference between it and the FLAC unless you have excellent hearing or it's being played on high-grade equipment.

So: FLAC -> mp3 OK mp3 -> FLAC Unnecessary

And mp3 <-> ogg vorbis: harmful.

But what would you gain from converting an MP3 to a FLAC?

Isn't that in the same vein as converting an existing 64kbps MP3 to a 256kbps one?

Well, at least with flac the sound won't change. It will just use a lot more disk space :)